The liquid in a glass thermometer, either colored alcohol or mercury, expands as it heats and condenses as it cools. In a thermometer, the liquid cannot expand outward and the only place it can go is up or down. Temperature on a glass thermometer can be measured in Fahrenheit, Celsius or Kelvin.
Once a thermometer is built and filled with liquid, numbers are printed, usually along the outside. These numbers make it easy to determine the temperature of a person, home or even the outdoors.
Thermometers that do not use liquid use a spring mechanism that expands as it heats and contracts as it cools.