What

The skin is the largest organ, covering 1.5–2 square meters in adults and weighing about 3.6 kg. It has three layers: epidermis, dermis, and hypodermis.

Light travels at exactly 299,792,458 meters per second in a vacuum — about 1.08 billion km/h. One light-year equals 9.46 trillion km, and sunlight takes 8 minutes 20 seconds to reach Earth.

DNA (deoxyribonucleic acid) is the molecule that stores genetic instructions in every living cell. Human DNA contains about 3 billion base pairs and is 99.9% identical between any two people.

The Challenger Deep in the Mariana Trench reaches 11,034 meters (36,201 feet) below sea level — deeper than Mount Everest is tall. Only three crewed descents have reached the bottom.

Venus is the hottest planet at 465°C (869°F) average surface temperature — hotter than Mercury despite being nearly twice as far from the Sun. Its thick CO₂ atmosphere creates a runaway greenhouse effect.

Mandarin Chinese has the most native speakers (~920 million), but English is the most widely spoken overall (~1.5 billion speakers including non-native). Spanish ranks third with ~550 million total speakers.

The Milky Way is our home galaxy — a barred spiral galaxy about 100,000 light-years in diameter containing 200–400 billion stars. Our Solar System sits about 26,000 light-years from the galactic center.

Vatican City is the smallest country by both area (0.44 sq km) and population (~800 citizens). It is an independent city-state entirely surrounded by Rome, Italy.

A black hole is a region where gravity is so strong that nothing, not even light, can escape. Its boundary is the event horizon; at the center lies a singularity.

Gravity is the attractive force between all objects with mass. Einstein described it not as a force but as the curving of spacetime caused by mass and energy.

Evolution is the change in the heritable traits of populations over successive generations, driven largely by natural selection. It explains the diversity of all life on Earth.

Absolute zero is the coldest possible temperature, −273.15°C (0 Kelvin), where atomic motion reaches its minimum. It can be approached but never fully reached.

The Higgs boson is a fundamental particle linked to the field that gives other particles mass. It was discovered at CERN in 2012, confirming a 1964 prediction.

Photosynthesis is how plants, algae, and some bacteria turn sunlight, water, and carbon dioxide into glucose and oxygen. It powers nearly all life on Earth.

A light-year is the distance light travels in one year — about 9.46 trillion kilometers. It is a measure of distance, not time.

The human genome is the complete set of DNA instructions for a person — about 3 billion base pairs and roughly 20,000 protein-coding genes. It was first fully sequenced in 2003.

Antimatter is made of particles with the opposite charge to ordinary matter. When matter and antimatter meet, they annihilate in a burst of pure energy.

The Fibonacci sequence is a series where each number is the sum of the two before it: 0, 1, 1, 2, 3, 5, 8, 13… It appears throughout nature and links to the golden ratio.

A supernova is the colossal explosion of a dying star, briefly outshining an entire galaxy and forging heavy elements.

An atom is the basic building block of matter, made of a dense nucleus of protons and neutrons surrounded by electrons.

A virus is a tiny infectious agent that can only reproduce inside a living cell. It sits at the boundary between living and non-living.

Climate change is the long-term shift in global temperatures and weather patterns, driven mainly by human greenhouse-gas emissions.

Electricity is the flow of electric charge, usually carried by electrons moving through a material such as a metal wire.

The immune system is the body defence network that detects and destroys harmful invaders like bacteria, viruses, and abnormal cells.

A galaxy is a vast system of stars, gas, dust, and dark matter bound together by gravity. The observable universe holds hundreds of billions of them.

Artificial intelligence is the field of building machines and software that can perform tasks that normally require human intelligence.

The water cycle is the continuous movement of water through evaporation, condensation, and precipitation across Earth.

A tsunami is a series of huge ocean waves, usually caused by an undersea earthquake, that can travel across whole oceans at jet speed.

A vaccine is a preparation that trains the immune system to recognise and fight a specific germ without causing the disease itself.

The periodic table is an organised chart of all known chemical elements, arranged by their atomic number and shared properties.

A comet is a small icy body that grows a glowing coma and tail when it nears the Sun and its ices vaporise.

Sound is a wave of vibrations travelling through a medium like air or water. It cannot travel through the vacuum of space.

A cell is the smallest unit of life. Every living thing is made of one or more cells, from single-celled microbes to humans.

A neutron star is the ultra-dense collapsed core of a massive star that exploded as a supernova.

Plate tectonics is the theory that Earth outer shell is divided into large moving plates whose interactions cause earthquakes, volcanoes, and mountain building.

An enzyme is a biological catalyst — usually a protein — that speeds up chemical reactions in living organisms without being consumed.

Dark matter is an invisible substance that makes up about 27% of the universe and exerts gravitational pull but does not emit or absorb light.

A prion is a misfolded protein that can cause other normal proteins to misfold, leading to fatal brain diseases.

The ozone layer is a region of the stratosphere, 15 to 35 km above Earth, where concentrated ozone (O₃) absorbs most of the Sun harmful UV-B and UV-C radiation.

The gut microbiome is the vast community of trillions of microorganisms living in the human digestive tract that aid digestion, immunity, and even mood.

A solar eclipse occurs when the Moon passes directly between the Sun and Earth, casting a shadow on our planet.

CRISPR is a gene-editing technology that allows scientists to precisely cut and modify DNA sequences in living organisms.

Plasma is the fourth state of matter — an ionised gas where atoms have lost electrons, making it electrically conductive.

Radiocarbon dating measures the decay of carbon-14 in organic material to estimate how long ago a living thing died.

Bioluminescence is the production of light by living organisms through a chemical reaction involving the molecule luciferin.

A semiconductor is a material whose electrical conductivity falls between that of a conductor and an insulator, and can be controlled by doping or voltage.

An aurora is a natural light display in the sky caused by charged particles from the Sun colliding with gases in Earth atmosphere.

A black body is an idealised object that absorbs all incoming electromagnetic radiation and re-emits it based solely on its temperature.

A quasar is an extremely luminous active galactic nucleus powered by a supermassive black hole devouring surrounding material.

An ecosystem is a community of living organisms interacting with each other and their physical environment as a functional unit.

The Doppler effect is the change in frequency of a wave as the source and observer move relative to each other.

A coral reef is an underwater ecosystem built by colonies of tiny animals (coral polyps) that secrete calcium carbonate skeletons over thousands of years.

A blockchain is a decentralised, tamper-resistant digital ledger that records transactions across many computers so no single entity controls the data.

A hormone is a chemical messenger produced by endocrine glands that travels through the blood to regulate distant organs and body processes.

A superconductor is a material that conducts electricity with zero resistance when cooled below a critical temperature.

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂ — carbon dioxide and water are converted into glucose and oxygen using sunlight.

Isotopes are variants of a chemical element that share the same number of protons but differ in their neutron count, giving them different atomic masses.

A neutrino is an electrically neutral, nearly massless subatomic particle that interacts with matter only through the weak nuclear force and gravity, making it extraordinarily difficult to detect.

The greenhouse effect is the natural warming of Earth surface caused by atmospheric gases trapping heat that would otherwise escape to space.

A meteoroid is a space rock; a meteor is the streak of light it creates in Earth atmosphere; a meteorite is the piece that survives to reach the ground.

The event horizon is the boundary around a black hole beyond which nothing — not even light — can escape.

Entropy is a measure of disorder or randomness in a system; the second law of thermodynamics states that entropy in an isolated system always increases.

A prism is a transparent optical element with flat, polished surfaces that refracts light, splitting white light into its constituent colours.

A pulsar is a rapidly spinning neutron star that emits beams of electromagnetic radiation from its magnetic poles; as the star rotates, these beams sweep across space like a lighthouse.

A tectonic plate is a massive slab of Earth lithosphere — crust and upper mantle — that floats on the semi-fluid asthenosphere and moves a few centimetres per year, driving earthquakes, volcanoes, and mountain building.

A supermassive black hole is a black hole with a mass of millions to billions of times that of the Sun, found at the centre of most large galaxies, including our Milky Way.

An alloy is a metallic material made by combining two or more elements (at least one a metal) to achieve properties — such as strength, corrosion resistance, or hardness — superior to the individual components.

A biome is a large-scale ecological community defined by its dominant vegetation, climate, and wildlife — examples include tropical rainforests, deserts, tundra, and grasslands.

Surface tension is the elastic-like force at the surface of a liquid, caused by cohesive forces between molecules pulling surface molecules inward, making the surface behave like a stretched membrane.

A catalyst is a substance that speeds up a chemical reaction without being permanently consumed, by lowering the activation energy needed for the reaction to proceed.

The Coriolis effect is the apparent deflection of moving objects (like wind and ocean currents) caused by Earth rotation — to the right in the Northern Hemisphere and to the left in the Southern.

Epigenetics is the study of heritable changes in gene expression that do not involve alterations to the DNA sequence itself — modifications that turn genes on or off without changing the genetic code.

Probiotics are live microorganisms — mainly bacteria and yeasts — that, when consumed in adequate amounts, confer a health benefit on the host by supporting the gut microbiome.

A dwarf planet is a celestial body that orbits the Sun, has sufficient mass for gravity to pull it into a roughly spherical shape, but has not "cleared the neighbourhood" around its orbit of other debris.

Absolute humidity is the total mass of water vapour present in a given volume of air, measured in grams per cubic metre, regardless of temperature.

A lichen is a composite organism formed by a symbiotic partnership between a fungus and one or more photosynthetic partners (green algae or cyanobacteria), functioning as a single ecological unit.

Permafrost is ground that remains frozen (at or below 0 degrees Celsius) continuously for at least two consecutive years, found beneath about 15% of the Northern Hemisphere land surface.

The Heisenberg uncertainty principle states that it is fundamentally impossible to simultaneously know both the exact position and exact momentum of a particle with arbitrary precision.

The aurora borealis (northern lights) is a natural light display caused by charged particles from the Sun colliding with atmospheric gases along Earth magnetic field lines near the poles, making oxygen and nitrogen glow.

Tardigrades ("water bears") are microscopic eight-legged animals, typically 0.1-1.5 mm long, famous for being the most resilient animals known — they can survive extreme temperatures, radiation, vacuum of space, and pressures that would destroy most life.

A piezoelectric material generates an electrical charge when mechanically stressed (squeezed, bent, or vibrated), and conversely changes shape when an electric field is applied — a direct coupling between mechanical and electrical energy.

Microplastics are tiny plastic particles less than 5 mm in diameter that result from the breakdown of larger plastic items, industrial processes, and synthetic textiles — they are now found in every environment on Earth, from deep ocean sediments to human blood.

A neutron bomb (enhanced radiation weapon) is a type of thermonuclear weapon designed to maximise lethal neutron radiation while minimising blast and thermal destruction — intended to kill personnel while leaving structures relatively intact.

Mycelium is the vast underground network of thread-like filaments (hyphae) that forms the main body of a fungus — mushrooms are just the visible fruiting bodies of this hidden network, which can span kilometres.

A rogue wave is an unexpectedly large ocean wave — at least twice the height of surrounding waves — that appears suddenly and can be catastrophically dangerous to ships, once dismissed as sailor myth but now scientifically confirmed.

A qubit (quantum bit) is the basic unit of quantum information, analogous to a classical bit but capable of existing in a superposition of both 0 and 1 simultaneously, enabling quantum computers to process certain problems exponentially faster.

Albedo is the fraction of incoming solar radiation that a surface reflects back into space, ranging from 0 (absorbs all light) to 1 (reflects all light) — it is a key factor in Earth energy balance and climate.

A geode is a rounded hollow rock lined with crystals (usually quartz or amethyst) on the inside — formed when mineral-rich water seeps into a gas bubble or cavity in volcanic or sedimentary rock and slowly deposits crystals over millions of years.

A keystone species is an organism that has a disproportionately large effect on its ecosystem relative to its abundance — its removal would cause dramatic changes or even collapse of the ecosystem.

Sonoluminescence is the mysterious phenomenon where sound waves in liquid cause tiny bubbles to collapse so violently that they emit brief flashes of light, reaching temperatures potentially exceeding 20,000 Kelvin.

A wetland is an area where water covers the soil or is present at or near the surface for at least part of the year — including marshes, swamps, bogs, and fens — making them among the most biologically productive ecosystems on Earth.

Electromagnetic induction is the generation of an electric current in a conductor by a changing magnetic field — the principle behind electric generators, transformers, and induction charging, discovered by Michael Faraday in 1831.

Fullerenes are hollow cage-shaped molecules made entirely of carbon atoms — the most famous being C60 (buckminsterfullerene or "buckyball"), a sphere of 60 carbon atoms arranged like a soccer ball, discovered in 1985.

Quantum entanglement is a phenomenon in which two or more particles become linked so that measuring the state of one instantly determines the state of the other, no matter how far apart they are.

The placebo effect is a real, measurable improvement in a person condition after receiving an inactive treatment, driven by their expectation and belief that the treatment will help.

A glacier is a large, persistent mass of ice that forms on land from compacted snow over many years and flows slowly under its own weight, reshaping landscapes as it moves.

A nebula is a vast interstellar cloud of gas and dust, often spanning many light-years, that can serve as a stellar nursery where new stars form or as the glowing remnant of a dying star.

Osmosis is the movement of water molecules across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration, equalising the concentration on both sides.

The Richter scale measures the magnitude of earthquakes on a logarithmic scale.

A meteor is the streak of light made when a space rock burns up in Earths atmosphere.

A hurricane is a powerful tropical storm with a spiralling structure and a calm central eye.

Inflation is the general rise in prices of goods and services over time, which reduces the purchasing power of money; a moderate, steady inflation rate (around 2%) is generally considered healthy for an economy.

Condensation is the process by which a gas turns into a liquid, such as water vapour forming droplets.

A fossil is the preserved remains or traces of a once-living organism from the distant past.

The pH scale measures how acidic or alkaline a solution is, running from 0 to 14.

A tsunami is a series of enormous ocean waves usually caused by underwater earthquakes, volcanic eruptions, or landslides that displace huge volumes of water; in the open ocean they are barely noticeable, but they grow into towering, destructive waves near shore.

A genetic mutation is a change in the DNA sequence of an organism; mutations can be harmful, beneficial, or neutral, and they are the ultimate source of the genetic variation that drives evolution by natural selection.

A tornado is a violently rotating column of air reaching from a thunderstorm to the ground.

A blood type is a classification of blood based on the presence or absence of specific molecules called antigens on the surface of red blood cells, most importantly the ABO and Rh systems.

A food chain is a sequence showing how energy and nutrients pass from one living thing to another as each organism eats the one before it, starting with producers like plants and moving up to predators.

A hologram is a three-dimensional image created by recording the pattern of light waves reflected from an object, so the image appears to have depth and changes as you view it from different angles.

A solar flare is a sudden, intense burst of radiation from the Sun, released when magnetic energy stored in its atmosphere is explosively unleashed, often near sunspots.

A tectonic fault is a fracture or zone of fractures in the Earth crust where blocks of rock have moved relative to each other; sudden slips along faults are what cause most earthquakes.

Carbon dating is a method for estimating the age of once-living material by measuring how much of the radioactive isotope carbon-14 remains, since it decays at a known, steady rate after death.

Cryptocurrency is a form of digital money that uses cryptography for security and typically runs on a decentralised ledger called a blockchain, allowing transactions without a central bank or government.

Dark energy is a mysterious form of energy thought to make up about 68 percent of the universe and to be responsible for driving its accelerating expansion.

Inflation is the general rise in prices over time, which reduces the purchasing power of money — meaning each unit of currency buys fewer goods and services than before.

Nuclear fusion is the process of forcing light atomic nuclei to merge into a heavier one, releasing enormous energy; it is the reaction that powers the Sun and other stars.

Radioactivity is the process by which unstable atomic nuclei lose energy by emitting radiation — such as alpha particles, beta particles, or gamma rays — as they transform into more stable forms.

Static electricity is an imbalance of electric charge that builds up on the surface of a material, usually created by friction, and is released as a spark or shock when it finds a path to discharge.

The electromagnetic spectrum is the full range of electromagnetic waves, from long-wavelength radio waves to short-wavelength gamma rays, with visible light making up only a tiny portion in the middle.

The internet is a vast global network of interconnected computers that communicate using shared protocols, allowing information to be exchanged between billions of devices around the world.

The jet stream is a fast, narrow band of high-altitude wind that flows from west to east around the planet, strongly influencing weather patterns and the routes of aircraft.

A caldera is a large, cauldron-like hollow that forms when a volcano collapses into its own emptied magma chamber after a massive eruption.

A coral polyp is a tiny soft-bodied marine animal that builds a hard skeleton; vast colonies of these polyps create coral reefs.

A cumulonimbus is a towering, dense storm cloud that can reach great heights and produces thunderstorms, heavy rain, lightning, hail, and sometimes tornadoes.

A fjord is a long, deep, narrow inlet of the sea carved by glaciers and flanked by steep cliffs, formed when the ocean floods a glacial valley.

A geyser is a hot spring that periodically erupts, shooting a column of boiling water and steam into the air when underground water is superheated by volcanic rock.

A moraine is a ridge or mound of rock and debris that a glacier picks up, carries, and deposits as it flows and later melts.

A limestone cave is an underground cavern dissolved out of limestone rock by slightly acidic water over thousands of years, often decorated with stalactites and stalagmites.

A lunar eclipse occurs when Earth passes directly between the Sun and the Moon, casting its shadow on the Moon and often turning it a coppery red.

A magnetar is a type of neutron star with an extraordinarily powerful magnetic field, the strongest known in the universe, capable of releasing enormous bursts of energy.

A mangrove forest is a coastal woodland of salt-tolerant trees that grow in tidal saltwater, with tangled roots that protect shorelines and shelter marine life.

A monsoon is a seasonal reversal of wind that brings a dramatic shift between wet and dry periods, most famously the heavy summer rains across South Asia.

A peninsula is a piece of land almost entirely surrounded by water but still connected to a larger landmass by a narrow strip called an isthmus.

A savanna is a tropical grassland biome dotted with scattered trees, defined by warm temperatures and a strong split between wet and dry seasons.

A sinkhole is a hole or depression in the ground that forms when underlying rock is dissolved away or an underground cavity collapses, sometimes suddenly and dramatically.

A stalactite is an icicle-shaped mineral formation that hangs from the ceiling of a cave, built up drop by drop from mineral-rich dripping water.

A storm surge is an abnormal rise of seawater pushed ashore by a storm strong winds and low pressure, often causing the deadliest flooding in hurricanes.

A tidal bore is a wave or wall of water that travels upstream against a river current when a strong incoming tide surges into a narrowing estuary.

The tundra is a cold, treeless biome with a short growing season and frozen subsoil called permafrost, found in the far north and on high mountains.

An oxbow lake is a curved, crescent-shaped lake formed when a loop of a meandering river is cut off from the main channel.

A blue moon is an extra full moon in a period, usually the second full moon in a single calendar month, and it is not actually blue in colour.

A VPN, or virtual private network, creates an encrypted tunnel between your device and a remote server, hiding your internet traffic from local networks and masking your IP address.

Cloud computing means renting computing power, storage and software over the internet from large data centres instead of owning and running the hardware yourself.

5G is the fifth generation of mobile network technology, offering much faster data speeds, lower delay and the ability to connect far more devices than 4G.

An IP address is a unique numerical label assigned to each device on a network, allowing data to be sent to the right place across the internet.

An algorithm is a precise, step-by-step set of instructions for solving a problem or completing a task, used by computers for everything from searching to recommending videos.

A recession is a significant, widespread decline in economic activity lasting months, often marked by falling output, rising unemployment and weaker spending.

The stock market is a network of exchanges where investors buy and sell shares, small pieces of ownership in public companies.

A large language model is an AI system trained on vast amounts of text to predict the next word in a sequence, allowing it to answer questions, write and summarise in natural language.

Machine learning is a branch of artificial intelligence in which computers learn patterns from data and improve at tasks without being explicitly programmed for each step.

Malware, short for malicious software, is any program designed to harm, spy on or take control of computers, including viruses, worms, ransomware and spyware.

Augmented reality overlays digital images, text or sounds onto a live view of the real world, usually through a phone camera or smart glasses.

GDP, or gross domestic product, is the total market value of all finished goods and services produced within a country in a given period, usually a year or quarter.

A credit score is a number that estimates how likely a person is to repay borrowed money, based on their history of loans, credit cards and bill payments.

An NFT, or non-fungible token, is a unique digital record on a blockchain that proves ownership of a specific item, such as a piece of digital art.

A pandemic is an outbreak of a new or re-emerging disease that spreads across multiple countries or continents, affecting a large number of people.

Cholesterol is a waxy, fat-like substance that the body needs to build cells and make hormones, but too much of certain types in the blood can clog arteries.

A calorie is a unit of energy; the food Calorie on nutrition labels is a kilocalorie, the energy needed to raise one kilogram of water by one degree Celsius.

Diabetes is a long-term condition in which blood sugar levels are too high because the body cannot make enough insulin or cannot use it properly.

A QR code is a square two-dimensional barcode that stores information such as a web link, which a phone camera can read instantly.

El Niño is a natural climate pattern in which the surface waters of the central and eastern tropical Pacific become unusually warm, shifting weather around the world.

The Big Bang is the scientific theory that the universe began about 13.8 billion years ago from an extremely hot, dense state and has been expanding and cooling ever since.

ADHD, or attention-deficit/hyperactivity disorder, is a common neurodevelopmental condition marked by ongoing patterns of inattention, hyperactivity and impulsivity.

A meme is an idea, joke or image that spreads from person to person and changes as it goes; the word was coined by biologist Richard Dawkins in 1976.

An API, or application programming interface, is a set of rules that lets one piece of software request data or services from another without knowing how it works inside.

Phishing is a scam in which criminals send fake emails, texts or calls pretending to be a trusted organisation to trick people into revealing passwords, money or personal information.

The keto diet is a very low-carbohydrate, high-fat eating plan that pushes the body into ketosis, a state where it burns fat and produces ketones for fuel instead of relying on sugar.

Intermittent fasting is an eating pattern that cycles between periods of eating and fasting, such as eating only within an eight-hour window each day.

Ozempic is a brand of semaglutide, a weekly injectable drug that mimics the gut hormone GLP-1 to lower blood sugar and reduce appetite; it is approved for type 2 diabetes, and a higher-dose version is sold for weight loss.

Dopamine is a chemical messenger in the brain that drives motivation, reward-seeking, learning and movement, rather than simply producing pleasure.

Black Friday is the day after Thanksgiving in the United States, traditionally the start of the Christmas shopping season and famous for huge discounts.

Compound interest is interest earned on both your original money and on the interest it has already earned, so savings or debts grow faster and faster over time.

Scientists describe romantic love as a set of brain systems for lust, attraction and attachment, driven by chemicals such as dopamine, oxytocin and vasopressin.

A migraine is a neurological condition that causes intense, often one-sided throbbing headaches lasting hours to days, frequently with nausea and sensitivity to light and sound.

The dark web is a small, hidden part of the internet that search engines cannot index and that can only be reached with special software such as the Tor browser, which hides users identities.

A deepfake is a video, image or audio clip created or altered with artificial intelligence to make someone appear to say or do something they never did.

The metaverse is the idea of shared, persistent 3D virtual worlds where people meet, work, play and trade, often using virtual or augmented reality.

IQ, or intelligence quotient, is a score from standardised tests of reasoning and problem-solving. Tests are set so the average score is 100.

Anxiety is the body’s natural response to perceived threat, causing worry, a racing heart and tension. When it is intense and persistent, it can become an anxiety disorder.

Gluten is a group of proteins found in wheat, barley and rye. It makes dough stretchy and helps bread rise.

Autism is a lifelong neurodevelopmental difference that affects how people communicate, interact and experience the world. It is a spectrum, so it varies widely from person to person.

Vitamin D is a nutrient the skin makes when exposed to sunlight. It helps the body absorb calcium and keeps bones and muscles healthy.

Gaslighting is a form of psychological manipulation in which someone makes another person doubt their own memory, perception or sanity.

Quantum computers use qubits that can exist in combinations of 0 and 1 at once. For certain problems, this could let them solve tasks far beyond ordinary computers.

Veganism is a way of living that avoids animal products, including meat, fish, dairy, eggs and honey, and often leather and wool too.

Stoicism is an ancient Greek philosophy that teaches people to focus on what they can control, accept what they cannot, and live by virtue.

Mindfulness means paying full attention to the present moment, on purpose and without judgment. It is often practised through meditation.

Narcissism is excessive self-focus, including a need for admiration and a lack of empathy. In its extreme form it is diagnosed as narcissistic personality disorder.

Sleep paralysis is being awake but temporarily unable to move or speak while falling asleep or waking up. It often comes with vivid, frightening hallucinations.

Umami is the fifth basic taste, alongside sweet, sour, salty and bitter. It is a savoury, meaty flavour caused mainly by glutamate.

A carbon footprint is the total amount of greenhouse gases caused by a person, product or organisation, usually measured in tonnes of carbon dioxide equivalent.

A hashtag is a word or phrase after a # symbol that groups social media posts on the same topic.

The Turing test checks whether a machine can hold a conversation indistinguishable from a human’s.

A leap second is an extra second occasionally added to clocks to keep atomic time in line with Earth’s rotation.

The Mandela effect is when many people share the same false memory of an event or detail.

FOMO, or fear of missing out, is anxiety that others are having rewarding experiences without you.

The butterfly effect is the idea that tiny changes in a complex system can lead to huge differences later.

It is a 1935 thought experiment showing how strange quantum rules seem when applied to everyday objects.

A mortgage is a loan used to buy property, where the property itself serves as security for the loan.

An index fund is an investment fund that tracks a market index, such as the S&P 500, instead of picking stocks.

A central bank manages a country’s money supply and interest rates and oversees the banking system.

It is a 2015 international treaty in which countries agreed to limit global warming to well below 2°C.

Net zero means cutting greenhouse gas emissions as far as possible and balancing the rest by removing an equal amount.

The polar vortex is a large area of cold, swirling air high above each pole that can spill south and bring freezing weather.

The Kuiper Belt is a vast ring of icy bodies beyond Neptune, home to dwarf planets such as Pluto.

A red giant is a dying star that has swollen to enormous size after running low on hydrogen in its core.

A white dwarf is the hot, dense core left behind after a Sun-like star dies.

An exoplanet is a planet that orbits a star other than our Sun.

A sonic boom is the thunder-like sound created by shock waves when an object travels faster than sound.

The golden ratio is about 1.618, a proportion where the whole relates to the larger part as the larger part relates to the smaller.

Pi (π) is the ratio of a circle’s circumference to its diameter, about 3.14159.

A prime number is a whole number greater than 1 that can be divided evenly only by 1 and itself.

Infinity is the idea of something without any end or limit, larger than any number.

A googol is 1 followed by 100 zeros, or 10 to the power of 100.

In a right-angled triangle, the square of the longest side equals the sum of the squares of the other two: a² + b² = c².

Cookies are small text files websites store in your browser to remember logins, settings and browsing activity.

Two-factor authentication (2FA) requires a second proof of identity, like a code or fingerprint, in addition to a password.

Ransomware is malicious software that locks or encrypts a victim’s files and demands payment to unlock them.

Open-source software has source code that anyone can freely view, use, modify and share.

A pixel is the smallest dot of colour that makes up a digital image or screen.

A gigabyte is a unit of digital storage equal to about one billion bytes.

A watt is the unit of power, measuring how fast energy is used or produced: one joule per second.

Serotonin is a chemical messenger involved in mood, sleep, digestion and many other body functions.

Melatonin is a hormone that signals darkness to the body and helps regulate the sleep–wake cycle.

Cortisol is a hormone made by the adrenal glands that helps the body respond to stress and regulates energy.

Adrenaline is a hormone released in moments of danger or excitement that prepares the body for fight or flight.

Body mass index (BMI) is weight in kilograms divided by height in metres squared, used as a rough health screen.

Dietary fibre is the part of plant foods the body cannot digest, which aids digestion and feeds gut bacteria.

It is an eating pattern rich in vegetables, fruit, whole grains, legumes, fish and olive oil, inspired by Mediterranean countries.

A concussion is a mild traumatic brain injury caused by a blow or jolt that shakes the brain inside the skull.

A stroke happens when blood supply to part of the brain is blocked or a blood vessel bursts, damaging brain cells.

Blood pressure is the force of blood pushing against artery walls, written as two numbers such as 120/80.

Sleep apnea is a disorder in which breathing repeatedly stops and starts during sleep.

Democracy is a system of government where power rests with the people, who rule directly or through elected representatives.

NATO is a military alliance of North American and European countries that pledge to defend each other.

The European Union is a political and economic union of 27 European countries sharing a single market and common rules.

A haiku is a short Japanese poem, traditionally of three lines with 5, 7 and 5 sound units.

A palindrome is a word, phrase or number that reads the same backwards as forwards.

Morse code represents letters and numbers with patterns of short and long signals, called dots and dashes.

The Great Red Spot is a giant storm on Jupiter, wider than Earth, that has raged for at least 190 years.

A wormhole is a theoretical tunnel through spacetime that could connect two distant points in the universe.

A phytoplankton bloom is a rapid population explosion of microscopic photosynthetic organisms in the ocean, visible from space as swirls of colour.

The Fibonacci sequence is a series of numbers where each number is the sum of the two preceding ones: 0, 1, 1, 2, 3, 5, 8, 13, 21, and so on.

Thermohaline circulation is a global ocean current system driven by differences in water temperature and salinity that moves heat around the planet.

A piezoelectric crystal generates an electric charge when mechanically stressed and deforms when an electric field is applied.

A coral atoll is a ring-shaped reef encircling a shallow lagoon, formed as coral grows around a volcanic island that gradually sinks.

A Leyden jar is the original electrical capacitor — a glass jar with metal foil inside and out that stores static electric charge.

A metamaterial is an engineered material with a structure designed to have properties not found in nature, such as bending light backward.

An allergy is the immune system overreacting to a normally harmless substance, such as pollen, pet dander, certain foods or dust mites.

A rip current is a narrow, fast-moving channel of water flowing seaward from the shore, caused by wave-driven water finding a return path.

A quorum is the minimum number of members who must be present for a legislative body to conduct official business.

A bioplastic is a plastic material derived partly or entirely from renewable biological sources like corn starch, sugarcane, or cellulose.

The photovoltaic effect is the process by which a material, typically a semiconductor like silicon, converts sunlight directly into electricity.

A tsunami warning system uses seismographs, ocean-floor pressure sensors and tide gauges to detect undersea earthquakes and abnormal waves and alert coastal populations.

The microbiome is the vast community of trillions of bacteria, viruses, fungi and other microorganisms that live in and on the human body, especially in the gut.

Trade winds are steady, tropical surface winds that blow from the northeast or southeast toward the equator.

A Lagrangian point is a position in space where the gravitational pull of two large bodies and centrifugal force balance, allowing a small object to orbit in a fixed relative position.

A smart grid is an electricity network that uses digital technology and two-way communication to efficiently manage power generation, distribution, and consumption.

An earthquake swarm is a series of many small to moderate earthquakes in one area over days to months, with no single dominant main shock.

A karst landscape is terrain shaped by the dissolution of soluble bedrock (usually limestone), featuring sinkholes, caves, and underground drainage.

A thermocline is a thin layer in a body of water where temperature changes more rapidly with depth than in the layers above or below.

A solstice is the moment when the Sun reaches its highest or lowest point in the sky at noon, giving the longest or shortest day of the year.

The polar vortex is a large area of low pressure and cold air spinning counterclockwise around the poles in the upper atmosphere.

A pangram is a sentence that uses every letter of the alphabet at least once, such as “The quick brown fox jumps over the lazy dog.”

A hadron is a subatomic particle made of quarks held together by the strong nuclear force — protons and neutrons are the most familiar examples.

A controlled (prescribed) burn is a planned fire set under specific conditions to reduce fuel loads, restore ecosystems, and prevent catastrophic wildfires.

RNA (ribonucleic acid) is a single-stranded molecule that reads and carries out the instructions encoded in DNA, serving as a messenger, builder and regulator in cells.

A moraine is a ridge or mound of unsorted rock, gravel, and sediment deposited by a glacier as it advances or retreats.

The gyroscopic effect is the tendency of a spinning object to resist changes to its axis of rotation and to precess when a torque is applied.

A black swan event is a rare, unpredictable occurrence with enormous impact that people tend to rationalise in hindsight, a concept popularised by Nassim Nicholas Taleb.

Ball lightning is a rare, unexplained atmospheric phenomenon appearing as a glowing, hovering sphere during thunderstorms.

Bioluminescence is the production of light by living organisms through a chemical reaction, seen in fireflies, deep-sea fish, jellyfish and some fungi.

A supercell is a severe, long-lived thunderstorm with a deep, persistently rotating updraft called a mesocyclone.

A Fresnel lens is a flat, lightweight lens made of concentric ridged sections that focus light as effectively as a much thicker conventional lens.

A bolide is an exceptionally bright meteor — a fireball that explodes or fragments in the atmosphere, often with a visible flash and sonic boom.

A penumbra is the partial shadow region where only some of the light source is blocked, producing a gradient between full shadow and full illumination.

A dead zone is an area of ocean or lake with so little dissolved oxygen that most marine life cannot survive, usually caused by excess nutrients from agriculture.

Doppler radar measures the velocity and direction of objects by detecting the frequency shift of reflected microwave pulses.

The Oort Cloud is a theoretical shell of icy objects surrounding the Solar System at up to about 1.5 light-years from the Sun, believed to be the source of long-period comets.

The placebo effect occurs when a patient’s condition improves after receiving an inert treatment, driven by expectation, conditioning and the brain releasing its own pain-relieving chemicals.

A blue ocean strategy is a business approach that creates uncontested market space by offering something so different that competition becomes irrelevant, rather than fighting in existing markets.

A zeolite is a microporous aluminosilicate mineral with a crystalline framework containing channels and cavities that selectively absorb molecules.

Coral bleaching happens when stressed corals expel the colourful algae living in their tissues, turning white and risking starvation if conditions don’t improve.

A tide pool is a small pocket of seawater left in rocky depressions along the shore when the tide goes out, creating a miniature marine ecosystem.

A bioluminescent bay glows blue-green at night due to massive concentrations of dinoflagellates that emit light when disturbed.

Herd immunity occurs when enough people in a population are immune to a disease, through vaccination or past infection, that its spread is greatly slowed, protecting those who cannot be vaccinated.

A Dyson sphere is a hypothetical megastructure that completely surrounds a star to capture most or all of its energy output, proposed by physicist Freeman Dyson in 1960.

The Fermi Paradox is the apparent contradiction between the high probability that extraterrestrial civilisations exist and the lack of any evidence or contact.

The urban heat island effect is the phenomenon where cities are significantly warmer than surrounding rural areas due to human activities and built surfaces.

A circadian rhythm is the roughly 24-hour internal clock that regulates sleep, hormones, body temperature and other processes in nearly all living things.

A fumarole is a vent in the Earth's surface that emits steam and volcanic gases from underground magma.

A supermoon occurs when a full moon coincides with the Moon being at or near its closest point to Earth (perigee), making it appear larger and brighter.

Rewilding is a conservation approach that restores natural processes and reintroduces missing species to allow ecosystems to manage themselves with minimal human intervention.

A refugee is someone who has been forced to flee their country because of war, persecution or violence and cannot safely return home.

Moore’s Law is the observation that the number of transistors on a microchip doubles roughly every two years, driving exponential growth in computing power.

A standing wave is a vibration pattern formed when two identical waves travelling in opposite directions interfere, creating fixed nodes and antinodes.

A red tide is a harmful algal bloom caused by rapid growth of toxic microscopic algae that can color the water red or brown.

The uncanny valley is the unsettling feeling people get when a robot or animation looks almost, but not quite, human.

A haboob is an intense dust storm pushed ahead of a thunderstorm's collapsing downdraft, forming a towering wall of dust.

A derecho is a long-lived, widespread windstorm produced by a fast-moving line of thunderstorms, with damage running in a straight path.

A lahar is a fast-flowing slurry of volcanic debris and water that rushes down the slopes of a volcano like wet concrete.

A pyroclastic flow is a ground-hugging avalanche of hot gas, ash and rock that races down a volcano at extreme speed.

An estuary is a partly enclosed body of water where a river meets the sea and fresh water mixes with salt water.

A mesa is an isolated, flat-topped hill with steep sides, left behind after surrounding land has eroded away.

A hydrothermal vent is a crack in the seafloor that spews mineral-rich water heated by magma below, often supporting unique ecosystems.

A brinicle is a downward-growing tube of ice that forms beneath sea ice when very cold, salty brine sinks into the ocean.

St. Elmo's fire is a glowing blue or violet electrical discharge that appears on pointed objects during stormy weather.

A moonbow is a rainbow produced by moonlight instead of sunlight, usually appearing pale or white to the naked eye.

The green flash is a brief burst of green light seen at the top of the Sun just as it sets or rises.

A sun dog is a bright spot of light that appears to one or both sides of the Sun, caused by ice crystals in the air.

Noctilucent clouds are the highest clouds on Earth, glowing electric blue in the twilight sky around 80 kilometres up.

A gamma-ray burst is an extremely powerful flash of gamma rays from a distant galaxy, the most energetic explosion known in the universe.

A brown dwarf is an object too massive to be a planet but too small to sustain hydrogen fusion like a true star.

The heliosphere is the vast bubble of charged particles and magnetic field blown out by the Sun, surrounding the entire Solar System.

Light pollution is excessive or misdirected artificial light that brightens the night sky and disrupts wildlife and human health.

Synesthesia is a neurological trait in which stimulating one sense automatically triggers experiences in another, such as seeing colours when hearing music.

Insomnia is a common sleep disorder in which a person has persistent trouble falling asleep, staying asleep or getting restful sleep.

Anemia is a condition in which the blood lacks enough healthy red blood cells or hemoglobin to carry adequate oxygen to the body.

Collagen is the most abundant protein in the human body, forming the strong fibres that hold together skin, bones, tendons and ligaments.

Keratin is a tough, fibrous protein that makes up hair, nails, the outer layer of skin, and animal horns, hooves, claws and feathers.

A neuron is a nerve cell that transmits information through electrical and chemical signals, forming the basic unit of the nervous system.

Hemoglobin is the iron-containing protein in red blood cells that carries oxygen from the lungs to the body and gives blood its red colour.

Esperanto is the most widely spoken constructed language, created in 1887 to be an easy, neutral second language for international communication.

A sonnet is a 14-line poem with a set rhyme scheme and rhythm, traditionally exploring themes like love, time and beauty.

Onomatopoeia is a word that imitates the sound it describes, such as buzz, sizzle, splash or cuckoo.

An oxymoron is a figure of speech that combines two contradictory words for effect, such as 'deafening silence' or 'bittersweet'.

Sign language is a complete natural language that uses hand shapes, movements, facial expressions and body posture instead of sound.

A hat-trick is when a single player achieves three successes in one game, such as three goals in football or three wickets in consecutive balls in cricket.

A player is offside if they are in the opponent's half and closer to the goal line than both the ball and the second-last defender when the ball is played to them.

A triathlon is an endurance race combining three consecutive sports: swimming, cycling and running.

Sumo is Japan's traditional full-contact wrestling sport, in which a wrestler wins by forcing his opponent out of the ring or making him touch the ground.

Origami is the Japanese art of folding paper into shapes and sculptures, traditionally without cutting or gluing.

Kintsugi is the Japanese art of repairing broken pottery with lacquer mixed with gold, making the cracks a beautiful part of the object's history.

Ikigai is a Japanese concept meaning 'a reason for being' — the sense of purpose that makes life feel worth living.

Hygge is a Danish and Norwegian word for a feeling of cosy contentment and togetherness, often enjoyed with candles, warm drinks and good company.

Zodiacal light is a faint, cone-shaped glow in the night sky caused by sunlight reflecting off dust spread through the inner Solar System.

A meteor shower is a celestial event in which many meteors appear to radiate from one point in the sky as Earth passes through debris left by a comet.

A geomagnetic storm is a major disturbance of Earth's magnetic field caused by bursts of charged particles from the Sun.

A double-blind trial is a scientific study in which neither the participants nor the researchers know who is receiving the real treatment and who is receiving a placebo.

A stem cell is an unspecialised cell that can divide to make more of itself and develop into specialised cells such as muscle, blood or nerve cells.

A chromosome is a tightly packed, thread-like structure of DNA and protein that carries genes inside the nucleus of a cell.

Metabolism is the full set of chemical reactions in the body that turn food into energy and build and repair tissues.

A filibuster is a tactic in a legislature where a member prolongs debate to delay or block a vote on a bill.

A referendum is a direct vote in which the entire electorate decides on a specific question or proposal rather than electing representatives.

A patent is a legal right granted by a government that gives an inventor the exclusive right to make, use and sell an invention for a limited time.

A Fabergé egg is one of the jewelled, intricately crafted Easter eggs made by the House of Fabergé for the Russian imperial family and others.

Parkour is a discipline of moving efficiently through an environment by running, jumping, climbing and vaulting over obstacles.

Hanukkah is the eight-day Jewish Festival of Lights, celebrating the rededication of the Second Temple in Jerusalem in the 2nd century BCE.

The Dunning-Kruger effect is a cognitive bias in which people with low skill in an area tend to overestimate their own ability.

Graphene is a single layer of carbon atoms arranged in a honeycomb pattern — one atom thick, incredibly strong and an excellent conductor.

The axolotl is a Mexican salamander that keeps its juvenile features, such as feathery gills, for life and can regrow lost body parts.

The Golden Record is a gold-plated copper disc carried by both Voyager spacecraft, holding sounds, music and images from Earth as a message to any alien finder.

The Gulf Stream is a powerful warm ocean current that flows from the Gulf of Mexico up the US East Coast and across the Atlantic towards Europe.

The Kármán line is a widely used boundary between Earth's atmosphere and outer space, set at an altitude of 100 kilometres.

A mirage is a real optical effect caused when light bends as it passes through layers of air at different temperatures, making distant objects appear displaced.

A Stradivarius is a stringed instrument, usually a violin, made by the Italian craftsman Antonio Stradivari, who worked in Cremona from the late 1600s to 1737.

A sourdough starter is a fermented mix of flour and water containing wild yeasts and lactic acid bacteria, used to make bread rise without commercial yeast.

An equinox is one of two moments each year, around 20 March and 22–23 September, when the Sun is directly above the equator and day and night are roughly equal everywhere.

An mRNA vaccine gives your cells temporary instructions to make a harmless piece of a germ's protein, training your immune system to recognise the real thing.

The Doomsday Clock is a symbolic clock set by the Bulletin of the Atomic Scientists to show how close humanity is to a global catastrophe, with midnight representing disaster.

Occam's razor is the principle that, among competing explanations, the one making the fewest assumptions should usually be preferred.

A narwhal's tusk is actually a giant tooth — usually the left canine — that spirals out through the upper lip and can grow up to about 3 metres long.

The overview effect is the profound shift in perspective many astronauts report after seeing Earth from space as a small, fragile, borderless planet.

Pangaea was a supercontinent that joined nearly all of Earth's land together about 335 to 200 million years ago before breaking apart into today's continents.

A Möbius strip is a loop with only one side and one edge, made by giving a strip of paper a half-twist before joining its ends.

A pangolin is a scaly, ant-eating mammal found in Asia and Africa — the only mammals covered in large protective scales made of keratin.

A marsupial is a mammal that gives birth to tiny, underdeveloped young, which usually continue growing inside their mother's pouch.

The Antikythera mechanism is an ancient Greek geared device, about 2,000 years old, that predicted astronomical positions and eclipses — often called the oldest known analogue computer.

The Voynich manuscript is a 15th-century illustrated book written in an unknown script that no one has convincingly deciphered.

A baker's dozen is 13 rather than 12. The most widely cited explanation is that medieval English bakers added an extra loaf to avoid penalties for selling short weight.

A Lagrange point is a spot in space where the gravity of two large bodies, like the Sun and Earth, balances a spacecraft's orbital motion so it can stay in position with little fuel.

A half-life is the time it takes for half of the atoms in a radioactive sample to decay. It ranges from fractions of a second to billions of years.

The haka is a traditional Māori ceremonial dance or challenge from New Zealand, performed with chanting, stamping, rhythmic movements and fierce facial expressions.

The stratosphere is the second major layer of Earth’s atmosphere, lying above the troposphere and below the mesosphere.

A supercontinent is a very large landmass formed when multiple continents join together into one.

A sequoia is a giant conifer tree species native to California, famous for being among the largest living trees on Earth.

A monarch butterfly is a bright orange-and-black butterfly known for one of the longest insect migrations in North America.

The platypus is a semi-aquatic mammal from eastern Australia that lays eggs and has a duck-like bill.

Lascaux is a famous cave in southwestern France known for its ancient Paleolithic paintings, especially of animals.

Stonehenge is a prehistoric stone monument in Wiltshire, England, built in several stages over many centuries.

The Panama Canal is a shipping canal in Panama that connects the Atlantic and Pacific Oceans.

The Sahara is the largest hot desert on Earth, covering much of North Africa.

The Danube is one of Europe’s major rivers, flowing through or past many countries before reaching the Black Sea.

A transistor is a semiconductor device that can amplify or switch electrical signals.

A microchip is a tiny piece of semiconductor material that contains electronic circuits.

The retina is the light-sensitive layer at the back of the eye that converts light into signals for the brain.

The diaphragm is a dome-shaped muscle beneath the lungs that plays a key role in breathing.

A cephalopod is a marine mollusk such as an octopus, squid, or cuttlefish.

A venomous snake is a snake that injects venom through specialized fangs to immobilize prey or defend itself.

A lighthouse is a tower or structure that emits light to guide ships and warn them about hazards.

A time capsule is a container filled with items meant to be opened in the future.

A world record is the best or most extreme officially recognized achievement in a particular category.

A lighthouse keeper was a person responsible for maintaining a lighthouse and keeping its light operating.

A telescope is an instrument that gathers and magnifies light to let us see distant objects more clearly.

A barcode is a machine-readable pattern of lines or squares that stores information about an item.

The International Space Station is a large habitable spacecraft that orbits Earth and serves as a research laboratory. It has been continuously occupied since November 2000.

The Pacific Ring of Fire is a horseshoe-shaped belt around the Pacific Ocean where many earthquakes and volcanoes occur. It is linked to plate boundaries and subduction zones.

The Suez Canal is a man-made waterway in Egypt that links the Mediterranean Sea and the Red Sea. It opened in 1869 and is a major shortcut for global shipping.

The Great Barrier Reef is the world’s largest coral reef system, off Australia’s northeast coast. It stretches for about 2,300 kilometers.

A koala is an Australian marsupial known for living in eucalyptus trees and eating mostly eucalyptus leaves. It is not a bear.

The Gutenberg printing press was a movable-type printing system developed in the 15th century. It helped make books far cheaper and faster to produce.

The Rosetta Stone is an ancient Egyptian stone slab inscribed with the same text in three scripts. It helped scholars decode hieroglyphs.

The Andes are the longest continental mountain range in the world, stretching along western South America. They run for about 7,000 kilometers.

Echolocation is a biological sonar used by bats to find objects and prey by listening to echoes. They emit high-frequency sounds beyond human hearing.

A black box flight recorder is a device on aircraft that records data about a flight. It helps investigators understand accidents and incidents.

A zero-day bug is a security flaw that is unknown to the vendor or has no available fix yet. Attackers can exploit it before a patch exists.

A snow leopard is a large cat native to the mountains of Central and South Asia. It is adapted to cold, steep habitats at high altitude.

The platypus is an egg-laying mammal from Australia with a duck-like bill and webbed feet. It is one of the few monotremes alive today.

The World’s Largest Ball of Twine is a roadside attraction in Cawker City, Kansas. It is a famous example of a novelty world record.

The Dead Sea is a salt lake bordered by Jordan and Israel/Palestine, famous for its very high salinity. Its water is so dense that people float easily.

The Bermuda Triangle is a loosely defined area in the western North Atlantic associated with stories of ship and aircraft disappearances. It is more legend than mystery in scientific terms.

A supernova remnant is the expanding cloud of gas and dust left behind after a star explodes. It can glow for thousands of years as the shockwave heats surrounding material.

A lighthouse beam is the directed light sent out by a lighthouse to guide ships. Its rotating pattern and flash interval help sailors identify the coast safely.

A neutron is a subatomic particle with no electric charge found in the nucleus of atoms. It has a mass similar to a proton and helps hold the nucleus together.

A mangrove is a salt-tolerant tree or shrub that grows in tropical and subtropical coastal waters. Mangroves protect shorelines and provide habitat for many species.

Echolocation is a way of locating objects by sending out sound waves and listening for the echoes. Bats and dolphins use it to navigate and hunt.

A supersonic aircraft is an airplane that flies faster than the speed of sound, or above Mach 1. It creates shock waves and often a sonic boom.

The ozone hole is a seasonal thinning of the ozone layer above Antarctica. It is caused mainly by human-made chemicals such as chlorofluorocarbons (CFCs).

A supercell is a long-lived, rotating thunderstorm. It is the most organized and often the most dangerous type of thunderstorm.

The Mariana Trench is the deepest known ocean trench on Earth. Its deepest point, Challenger Deep, lies in the western Pacific Ocean.

The Amazon rainforest is the world’s largest tropical rainforest. It covers much of the Amazon Basin in South America and is known for extraordinary biodiversity.

The hippocampus is a brain structure important for memory and spatial navigation. It is part of the limbic system in the medial temporal lobe.

The cornea is the clear front surface of the eye. It helps focus light onto the retina and protects the eye from dust and germs.

An octopus is a soft-bodied marine mollusk with eight arms. It is known for intelligence, camouflage and the ability to squeeze through tiny spaces.

A kangaroo is a large marsupial native to Australia. It carries its young in a pouch and moves by hopping on powerful hind legs.

Machu Picchu is a 15th-century Inca citadel in the Andes of Peru. It is one of the most famous archaeological sites in the world.

The transcontinental railroad was the U.S. railway line that connected the East Coast rail network to the Pacific Coast. It was completed in 1869.

A quokka is a small marsupial native to southwestern Australia. It is famous for its friendly-looking “smile” and relaxed behavior.

The Nobel Prize is a set of prestigious international awards given for outstanding achievements. It was established by the will of Alfred Nobel and first awarded in 1901.

A time zone is a region of the Earth that uses the same standard time. Time zones help coordinate daily life across different longitudes.

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to make sugar and release oxygen.

A moat is a ditch, often filled with water, built around a castle or fortress as a defensive barrier.

The Hubble Space Telescope is a space telescope launched in 1990 that has produced some of the most detailed images of the universe.

A supervolcano is a volcanic system capable of erupting at least 1,000 cubic kilometers of material, far larger than typical eruptions. These eruptions can cause global climate effects.

A DNA fingerprint is a pattern of DNA markers used to identify a person with very high confidence. It relies on variations in specific regions of the genome that differ between individuals.

A geyser cone is the mound or hill of mineral deposits built around some geysers by repeated eruptions. It forms from silica or other dissolved minerals left behind by hot water.

The International Date Line is an imaginary line near the 180° meridian where the calendar date changes by one day. Crossing it westward adds a day, and crossing eastward subtracts a day.

A leap year is a year with 366 days instead of 365, adding February 29. It keeps the calendar aligned with Earth’s orbit around the Sun.

An aquifer is an underground layer of rock, sand, or gravel that stores and transmits groundwater. Many wells and springs draw water from aquifers.

A quipu is an Inca recording system made of knotted cords used for accounting and possibly more. It was used in the Andes before and during the Spanish conquest.

A watershed is an area of land where all water drains to a common outlet, such as a river, lake, or ocean. It can also mean the dividing line between drainage basins.

A rhino horn is a structure made mostly of keratin, the same protein found in human hair and nails. It is attached to the skin, not the skull bone, and grows continuously.

A viaduct is a bridge-like structure made of a series of arches, spans, or columns carrying a road or railway across a valley, river, or other obstacle. It is often much longer than a single bridge.

A bard is a poet, storyteller, or singer, especially in ancient Celtic traditions. Bards preserved history, genealogy, and heroic tales through oral performance.

A salt flat is a broad, flat area covered with salt and other minerals left behind when water evaporates. The Salar de Uyuni in Bolivia is the largest salt flat on Earth.

A trebuchet is a medieval siege engine that uses a counterweight to launch projectiles. It was more powerful and accurate than many earlier catapults.

Sonar is a system that uses sound waves to detect and locate objects underwater. The name stands for Sound Navigation and Ranging.

A biomechanical prosthesis is an artificial body part that combines mechanical structure with biological or electronic control. It is designed to restore function after limb loss or injury.

A hygrometer is an instrument that measures humidity, or the amount of water vapor in the air. It is used in weather monitoring, homes, museums, and industry.

A sepulchre is a burial place, tomb, or small chamber for the dead. The term is often used in historical, religious, and architectural contexts.

A geyser plume is the cloud of hot water droplets, steam, and dissolved minerals thrown into the air during a geyser eruption. It can rise high above the vent before falling back as spray.

A boomerang is a curved throwing tool, traditionally used by Indigenous Australians for hunting and sport. Some boomerangs are designed to return to the thrower, while others are not.

A semaphore signal is a visual communication system that uses positions of flags or arms to represent letters and transmit messages at a distance.

A narwhal is an Arctic whale best known for the long spiral tusk found on males, which is actually an elongated tooth.

A sodium-ion battery is a rechargeable battery that uses sodium ions to store and release electricity, offering an alternative to lithium-ion technology.

The pancreas is an organ behind the stomach that helps with both digestion and blood sugar control. It releases digestive enzymes into the small intestine and hormones such as insulin into the bloodstream.

The continental shelf is the shallow underwater edge of a continent that extends from the shoreline to the point where the seafloor drops off steeply. It is usually covered by relatively shallow seas.

The Great Rift Valley is a vast network of valleys and escarpments in East Africa formed by tectonic plate movement. It stretches for thousands of kilometers from the Middle East down through eastern Africa.

The hypotenuse is the longest side of a right triangle, opposite the right angle. In a right triangle, it is related to the other two sides by the Pythagorean theorem.

A firewall is a security system that monitors and controls network traffic based on set rules. It helps block unauthorized access to computers and networks.

A monorail is a railway system in which the train runs on a single rail or beam. It is often used for airport links, urban transit, or amusement attractions.

The femur is the thigh bone and the longest, strongest bone in the human body. It runs from the hip to the knee.

A komodo dragon is the world’s largest living lizard, native to a few Indonesian islands. Adults can reach about 3 meters in length and weigh well over 70 kilograms.

A plateau is a broad area of high land with a mostly flat top. It rises sharply above surrounding land and can be formed by uplift, lava, or erosion.

The steam engine is a machine that uses expanding steam to do mechanical work. It became a key technology of the Industrial Revolution.

Mount Kilimanjaro is Africa’s highest mountain, rising to 5,895 meters above sea level in Tanzania. It is a dormant volcano with three main cones.

A centipede is a long, many-legged arthropod that belongs to the class Chilopoda. Despite the name, centipedes do not have exactly 100 legs.

A steam engine is a machine that uses steam pressure to do work, usually by moving a piston or turbine. It was a key invention of the Industrial Revolution.

A genome is the complete set of genetic material in an organism, including all of its DNA and genes.

The diaphragm is a dome-shaped muscle beneath the lungs that is the main muscle used for breathing.

A clockwork mechanism is a system of gears, springs, and escapements that stores and releases energy in a controlled way to keep time or power moving parts. It is the basis of mechanical clocks and many wind-up devices.

The diaphragm is a large dome-shaped muscle beneath the lungs that plays the main role in breathing. When it contracts, the chest cavity expands and air is drawn into the lungs.

A mesocyclone is a rotating column of air inside a thunderstorm, usually within a supercell. It is one of the key features that can lead to tornado formation.

A methane clathrate is an ice-like solid in which methane molecules are trapped inside cages of water molecules. It is found in deep ocean sediments and under permafrost.

The aurora australis is the Southern Hemisphere's southern lights, a natural glow in the sky caused by charged particles from the Sun interacting with Earth's atmosphere.

The thermocline is a layer in a lake or ocean where temperature changes rapidly with depth.

The continental shelf is the shallow underwater edge of a continent that stretches from the coast to the continental slope.

A gravity assist is a maneuver that uses a planet’s motion and gravity to change a spacecraft’s speed and direction without much fuel. It is also called a slingshot maneuver.

A coronal mass ejection, or CME, is a huge cloud of charged particles thrown out from the Sun. If aimed at Earth, it can trigger geomagnetic storms and auroras.

The solar wind is a continuous stream of charged particles flowing outward from the Sun. It shapes comet tails and helps create auroras when it reaches Earth.

An exomoon is a moon that orbits a planet outside our Solar System. It is the extraterrestrial equivalent of Earth’s Moon.

A circumbinary planet is a planet that orbits two stars instead of one. It circles around both stars as they orbit each other.

A hot Jupiter is a gas giant exoplanet that orbits very close to its star and has extremely high temperatures. Many complete an orbit in just a few days.

The transit method finds exoplanets by measuring the tiny dip in a star’s brightness when a planet passes in front of it. Repeating dips can reveal the planet’s size and orbit.

A light curve is a graph showing how an object’s brightness changes over time. Astronomers use it to study stars, planets, supernovae, and eclipsing systems.

A neutron star merger happens when two neutron stars spiral together and collide. It can produce gravitational waves and a kilonova explosion.

A kilonova is a bright explosion created when neutron stars merge. It is powered by radioactive decay of heavy elements formed in the event.

The James Webb Space Telescope is a large infrared space telescope launched in 2021 to study the early universe, stars, and planets. It is the most powerful space observatory ever sent into orbit at infrared wavelengths.

The Sun-Earth L2 point is a stable-ish location about 1.5 million kilometers from Earth on the opposite side of the Sun. Spacecraft placed there can keep the Sun, Earth, and Moon mostly on one side.

A launch window is the specific time period when a rocket can launch to reach its target efficiently and safely. It depends on orbital alignment, weather, and mission constraints.

A flyby mission sends a spacecraft past a planet, moon, asteroid, or comet without entering orbit or landing. It is a fast way to gather close-up data on many targets.

A rover is a robotic vehicle designed to move across the surface of another world. It can study rocks, soil, atmosphere, and geology in places that are too dangerous for humans.

A sample return mission collects material from another world and brings it back to Earth for detailed analysis. It lets scientists study the material with the full range of Earth laboratories.

A retrograde orbit is an orbit in which an object moves opposite to the rotation of the body it circles. Most natural moons orbit prograde, so retrograde motion is less common.

A circumpolar star is a star that never sets below the horizon for a particular location on Earth. It stays above the horizon all year long.

A Trojan asteroid is a small body that shares an orbit with a planet, usually clustered near stable Lagrange points. Jupiter has the most famous Trojan swarms.

An armillary sphere is a historical astronomical instrument made of rings that model the celestial sphere. It was used to visualize the positions of stars and planets.

An observatory is a facility built to observe celestial objects or space phenomena. It may contain telescopes, detectors, and instruments on the ground or in space.

The Parker Solar Probe is a NASA spacecraft designed to study the Sun up close. It has flown closer to the Sun than any previous spacecraft and samples the corona directly.

A terrestrial planet is a rocky planet with a solid surface, like Mercury, Venus, Earth, and Mars.

The asteroid belt is a region between Mars and Jupiter where many rocky objects orbit the Sun.

Orbital resonance happens when orbiting bodies exert regular, repeating gravitational pulls on each other because their orbital periods are linked.

A synodic period is the time it takes for a planet or moon to return to the same position relative to another object and the Sun.

A retrograde planet is one that appears to move backward in the sky for a time because of relative orbital motion.

A planet’s albedo is the fraction of sunlight it reflects back into space.

A planet’s axial tilt is the angle between its rotation axis and the perpendicular to its orbital plane.

A magnetosphere is the region around a planet dominated by its magnetic field.

A meteoroid is the space rock, a meteor is the streak of light it makes in the atmosphere, and a meteorite is what reaches the ground.

A crater is a bowl-shaped depression formed by impact from a meteoroid, asteroid, or comet.

Regolith is the loose layer of dust, soil, and broken rock covering a planet, moon, or asteroid.

A gas giant is a large planet made mostly of hydrogen and helium, with no well-defined solid surface.

An ice giant is a large planet with a composition rich in volatile ices such as water, ammonia, and methane.

Neptune’s Great Dark Spot was a large storm system observed by Voyager 2 in 1989.

Titan is Saturn’s largest moon and the only moon in the Solar System with a thick atmosphere.

Europa is a moon of Jupiter that likely has a subsurface ocean beneath its icy crust.

A subsurface ocean is a layer of liquid water hidden beneath ice or rock on a moon or dwarf planet.

Enceladus is a small icy moon of Saturn that shoots water-rich plumes from its south pole.

A ring system is a set of countless small particles orbiting a planet in a flat, disk-like formation.

Cosmic microwave background radiation is the faint afterglow of the early Universe, visible in every direction. It comes from about 380,000 years after the Big Bang.

Redshift is the stretching of light to longer wavelengths, making objects look redder than they really are. In astronomy it is a key clue that distant galaxies are moving away.

An active galactic nucleus, or AGN, is a compact region at the center of a galaxy that emits huge amounts of energy. It is powered by matter falling onto a supermassive black hole.

Gravitational lensing is the bending of light by mass, such as galaxies or clusters of galaxies. It can magnify distant objects and even create arcs or multiple images.

An Einstein ring is a nearly perfect circular image made when gravitational lensing bends light from a distant object around a massive foreground object. It is a rare alignment effect.

The Hubble constant is the number that describes how fast the Universe is expanding per unit distance. It is usually given in kilometers per second per megaparsec.

Cosmic inflation is the idea that the early Universe underwent a brief period of extremely rapid expansion. It was proposed in the early 1980s to explain several features of the cosmos.

Baryogenesis is the process that may have produced the small excess of matter over antimatter in the early Universe. Without it, the Universe could have ended up mostly as radiation.

The Tolman test is a way of checking whether the Universe is expanding by looking at how surface brightness changes with distance. In an expanding Universe, distant objects should appear much dimmer than a static-universe model predicts.

Cosmic shear is the tiny distortion in the shapes of distant galaxies caused by weak gravitational lensing. It helps astronomers map dark matter across large scales.

A galaxy filament is a long, threadlike structure in the cosmic web made of galaxies, gas, and dark matter. Filaments connect clusters and superclusters across enormous distances.

A void is a huge region of space with far fewer galaxies than average. Voids are a major part of the cosmic web.

A proto-galaxy is an early stage in galaxy formation, before a galaxy has fully assembled into its final shape. These objects are found in the young Universe.

A starburst galaxy is a galaxy forming stars at an unusually high rate. Its star formation can be dozens or even hundreds of times faster than in calmer galaxies.

A supercluster is a very large grouping of galaxy clusters and groups. It is one of the biggest structures recognized in the observable Universe.

A Type Ia supernova is a powerful stellar explosion that happens when a white dwarf undergoes runaway fusion. It produces a very bright and relatively standard peak brightness.

A standard candle is an object whose true brightness is known, allowing astronomers to estimate distance by comparing it with apparent brightness. Type Ia supernovae are a famous example.

A Cepheid variable is a star whose brightness changes in a regular pattern as it expands and contracts. Its pulsation period is directly related to its true luminosity.

The cosmic distance ladder is a chain of methods astronomers use to measure distances from nearby objects to faraway galaxies. Each step calibrates the next one.

Stellar parallax is the apparent shift in a nearby star’s position when viewed from different points in Earth’s orbit. It is the simplest direct way to measure nearby stellar distances.

A star’s spectral class is a category based on its temperature and the lines seen in its spectrum. The main classes are O, B, A, F, G, K, and M.

The Hertzsprung-Russell diagram is a graph that plots stars by luminosity and temperature. It reveals major stages of stellar evolution.

The initial mass function describes how many stars form at different masses in a star-forming region. It helps astronomers understand the mix of small and large stars in galaxies.

Reionization was the era when the first stars and galaxies ionized the hydrogen gas in the Universe again. It ended the cosmic dark ages and began the transparent Universe we see today.

CMB anisotropy is the tiny variation in temperature across the cosmic microwave background. These small differences are the fingerprints of the early Universe’s density fluctuations.

A galaxy rotation curve is a plot of how orbital speed changes with distance from a galaxy’s center. Unexpected flat rotation curves are one of the key clues for dark matter.

A stellar nursery is a cloud of gas and dust where new stars are born. These regions are often called nebulae or molecular clouds.

A globular cluster is a tightly packed spherical group of very old stars orbiting a galaxy. Some globular clusters contain hundreds of thousands or even millions of stars.

The cosmic web is the large-scale network of filaments, clusters, and voids that makes up the Universe’s structure. It reflects how matter was arranged by gravity over billions of years.

The weak nuclear force is one of the four fundamental forces and is responsible for certain kinds of particle decay, including beta decay. It acts over a very short range inside atomic nuclei.

The strong nuclear force binds quarks inside protons and neutrons, and it also holds atomic nuclei together. It is the strongest of the four fundamental forces but acts only at extremely short distances.

The electromagnetic force is the interaction between electric charges and magnets. It explains electricity, magnetism, light, and most everyday chemical and material behavior.

Gravity is the attractive force between masses. It gives objects weight, keeps planets in orbit, and shapes galaxies and the large-scale structure of the universe.

A gauge boson is a particle that carries one of the fundamental forces in nature. Examples include photons, gluons, and the W and Z bosons.

A virtual particle is a temporary quantum entity used to describe interactions between particles. It is not directly observable as a free particle like an electron or photon.

The Standard Model is the theory that describes the known fundamental particles and three of the four fundamental forces. It does not include gravity.

The Higgs field is an all-pervading quantum field that gives certain particles mass through their interaction with it. It is central to the Standard Model.

Quantum field theory is the framework that combines quantum mechanics with special relativity to describe particles as excitations of fields. It is the foundation of modern particle physics.

A Feynman diagram is a visual tool used to calculate and picture particle interactions in quantum field theory. It shows how particles exchange force carriers.

Beta decay is a type of radioactive decay in which a neutron changes into a proton, or a proton into a neutron. It is caused by the weak nuclear force.

Radioactive decay is the spontaneous change of an unstable atomic nucleus into a more stable one. It can occur through alpha, beta, or gamma decay.

A half-life is the time it takes for half of a radioactive sample to decay. It is a characteristic property of a specific unstable isotope.

Nuclear fission is the splitting of a heavy atomic nucleus into smaller nuclei, releasing energy and neutrons. It is the process used in nuclear reactors and atomic weapons.

Inertia is an object’s tendency to resist changes in its motion. It is closely related to mass.

Momentum is the quantity of motion an object has, equal to its mass multiplied by its velocity. It is conserved in isolated systems.

Torque is the turning effect of a force around a pivot or axis. It is what makes objects rotate.

The lever arm is the perpendicular distance from a pivot point to the line of action of a force. It determines how much torque the force produces.

Resonance is the large increase in vibration that happens when a system is driven at its natural frequency. It can amplify motion, sound, and even structural stress.

Polarization is the orientation of light’s oscillating electric field. It is a property of transverse electromagnetic waves.

The photoelectric effect is the emission of electrons from a material when it absorbs light of sufficiently high frequency. It helped establish the quantum nature of light.

The Lorentz force is the force on a charged particle moving through electric and magnetic fields. It explains how charges move in electromagnetic fields.

The Casimir effect is a tiny attractive force between very closely spaced uncharged conductive plates. It arises from quantum fluctuations in the vacuum.

The equivalence principle says that gravitational and inertial mass are equivalent, and that free fall is locally indistinguishable from weightlessness. It is a cornerstone of general relativity.

The atomic number is the number of protons in an atom’s nucleus. It defines which element the atom is.

Atomic mass is the average mass of an element’s atoms, taking isotopes into account. It is usually shown in atomic mass units on the periodic table.

Electronegativity is an atom’s tendency to attract shared electrons in a chemical bond. Fluorine is the most electronegative element.

An ion is an atom or molecule that has gained or lost electrons and therefore carries a charge. Positive ions are cations, and negative ions are anions.

A covalent bond is a chemical bond where atoms share one or more pairs of electrons. It is common between nonmetals.

An ionic bond is the attraction between oppositely charged ions. It usually forms when one atom transfers electrons to another.

A valence electron is an electron in the outermost shell of an atom. These electrons mainly control how an element reacts chemically.

A noble gas is one of the elements in Group 18 of the periodic table. These gases are very unreactive because their outer electron shells are full.

An alkali metal is a Group 1 element such as lithium, sodium, or potassium. These metals are highly reactive because they have one valence electron.

A halogen is a Group 17 element on the periodic table. Halogens are very reactive nonmetals that often form salts with metals.

A transition metal is an element in the central block of the periodic table, usually Groups 3 through 12. Many transition metals are known for multiple oxidation states and colored compounds.

A metalloid is an element with properties between metals and nonmetals. Common metalloids include silicon and germanium.

pH is a scale that measures how acidic or basic a solution is. It is based on the concentration of hydrogen ions.

An acid is a substance that increases the concentration of hydrogen ions in water. Acids typically have a pH below 7.

A base is a substance that can accept hydrogen ions or release hydroxide ions in water. Bases usually have a pH above 7.

The lanthanides are a group of 15 metallic elements from lanthanum to lutetium. They are often called the rare earth elements, though many are not especially rare in the crust.

The actinides are 15 elements from actinium to lawrencium, shown in a separate row below the periodic table. Many actinides are radioactive.

Allotropy is when an element exists in two or more different structural forms in the same physical state. Carbon and oxygen are famous examples.

A crystal lattice is a regular, repeating arrangement of atoms, ions, or molecules in a solid. It helps explain why crystals have definite shapes and properties.

A mole is a counting unit used in chemistry, equal to about 6.022 × 10^23 particles. It lets chemists connect tiny atoms to measurable amounts of पदार्थ.

Stoichiometry is the part of chemistry that deals with the quantitative relationships in chemical reactions. It tells you how much reactant is needed and how much product will form.

Oxidation is a chemical process in which a substance loses electrons. In everyday language it often refers to reactions with oxygen, like rusting.

A redox reaction is a chemical reaction in which electrons are transferred between substances. It includes both oxidation and reduction.

Electrolysis is the use of electricity to drive a chemical reaction that would not happen on its own. It is commonly used to break compounds apart.

Corrosion is the gradual damage or deterioration of a material, usually a metal, through chemical or electrochemical reactions. Rusting is the best-known example.

An unconformity is a gap in the rock record, where younger rocks sit on top of older rocks after erosion or non-deposition. It shows that a long span of time is missing from the geologic sequence.

Varves are annual layers of sediment, usually deposited in lakes. Each pair of light and dark layers can represent one year of deposition.

Loess is a blanket of fine silt-sized sediment blown by wind and deposited on land. It is often very fertile but can also be highly erosion-prone.

A karst spring is a place where groundwater emerges from limestone or other soluble rock terrain. It often occurs in regions shaped by caves, fractures, and underground drainage.

A joint is a natural crack in rock that has not moved significantly along the fracture. Joints help control how rocks break, weather, and form cliffs or blocks.

A fault scarp is a steep slope or cliff created by movement along a fault. It marks where the ground has been vertically displaced by tectonic forces.

An anticline is an upward-arching fold in layered rock. In a typical anticline, the oldest rocks are found near the center of the fold.

A syncline is a downward-arching fold in layered rock. In a typical syncline, the youngest rocks are found near the center.

A batholith is a huge mass of intrusive igneous rock that crystallized deep underground. It is usually larger than 100 square kilometers at the surface exposure.

Pumice is a light, porous volcanic rock filled with gas bubbles. So many bubbles can be trapped in it that some pieces float on water.

Obsidian is volcanic glass formed when lava cools so rapidly that crystals do not have time to grow. It is usually dark and breaks with a sharp, glassy fracture.

Pillow lava is basaltic lava that cools underwater into rounded, pillow-shaped masses. The outer skin hardens quickly while the inside keeps flowing.