A thermometer is an instrument that measures temperature: how hot or cold something is. The word comes from Greek roots meaning “heat measure.”

Most traditional thermometers contain a thin tube of liquid (usually mercury or colored alcohol) that expands when it gets warmer and contracts when it cools down. As the liquid moves up or down the tube, it points to numbers on a scale showing the temperature in degrees. When you're sick and someone checks if you have a fever, they're using a thermometer to see if your body temperature is higher than the normal 98.6 degrees Fahrenheit.

Digital thermometers work differently, using electronic sensors instead of liquid, but they serve the same purpose: telling you exactly how hot or cold something is. Weather forecasters use outdoor thermometers to report daily temperatures. Cooks use thermometers to check if meat is cooked safely. Scientists use specialized thermometers to measure everything from the freezing Arctic to the heat of chemical reactions.

Thermometers gave us precise numbers, which proved crucial for science, medicine, and understanding our world.