How does a microwave oven work?

At the heart of a microwave oven is a device called a magnetron, which generates microwaves — a form of electromagnetic radiation — typically at a frequency of about 2.45 gigahertz. These waves penetrate the food and are absorbed mainly by water molecules, making them flip back and forth billions of times per second. That frantic molecular motion is heat, so the food warms from the inside as well as the surface. A metal mesh in the door reflects the microwaves back inside, keeping them contained. The oven was discovered by accident in 1945 when engineer Percy Spencer noticed a chocolate bar melting in his pocket near radar equipment.