How does a thermometer work?

Traditional liquid-in-glass thermometers contain mercury or coloured alcohol, which expands as it warms and rises up a narrow tube marked with a scale. Digital thermometers usually use a thermistor, a component whose electrical resistance changes with temperature, which a chip converts into a reading. Infrared thermometers, like forehead and ear models, detect the invisible infrared energy that all warm objects give off. Bimetallic strips, made of two metals that expand at different rates, bend with temperature and are used in dial thermometers and thermostats. Galileo built an early thermoscope around 1593, and Daniel Fahrenheit made the first mercury thermometer in 1714.