Thermometer

A thermometer is a tool that measures temperature. Temperature tells you how hot or cold something is. Thermometers are used to check the weather, to take a sick person's temperature, to cook food safely, and to run science experiments. Without them, scientists could not measure heat in any careful way.

Most thermometers work because things expand when they get warmer and shrink when they get cooler. An old-style thermometer is a thin glass tube with liquid inside, usually colored alcohol. When the air around the tube warms up, the liquid grows a little and climbs higher. When the air cools, the liquid shrinks and drops. Numbers along the tube show the temperature.

The first real thermometer was invented around 1600 by the Italian scientist Galileo Galilei. Better versions came later. For a long time, many thermometers used mercury, a silvery liquid metal. Mercury works well but is poisonous, so most countries have stopped using it.

Today, many thermometers are digital. A tiny sensor inside measures temperature and shows the number on a screen. Some thermometers can even measure heat from far away by reading the invisible light that warm objects give off. A doctor can point one at your forehead and read your temperature in about one second, without touching your skin at all.