Temperature describes how hot or cold something is. An object’s temperature represents its average thermal energy—how fast or slowly its atoms or molecules are moving. Water molecules of boiling water, for instance, move much faster than frozen water molecules, so boiling water has more thermal energy. This is why boiling water has a higher temperature than ice.

Temperature is measured in three different ways. Most of the world measures temperature with degrees Celsius, which is part of the metric system. In this system, 0°C marks water’s freezing temperature and 100°C marks its boiling point. Most scientists use the Celsius scale when measuring temperatures in experiments.

The United States and a few other countries still use the Fahrenheit scale for nontechnical purposes, such as TV weather reports. In this system, 32°F marks the temperature at which water freezes and 212°F marks its boiling point.

The third system is predominantly used by scientists and is called the kelvin scale. It is based on the coldest temperature possible—absolute zero—which is equal to 0 kelvin. In this system, water freezes at 273.15K and boils at 373.15K. The degree symbol is not used when expressing temperatures in the kelvin scale.

Humans are not very good at gauging temperature. For example, if a person were to simultaneously touch a book and a metal pipe that are both at room temperature, the metal pipe would feel cooler. That is because metal is a better conductor of heat than paper, so a metal pipe draws heat away from the skin faster than a book. That makes the pipe feel cooler than the book even though it is exactly the same temperature.

Temperature is measured with a thermometer. The liquid thermometer was one of the earliest types of thermometer. These instruments measure temperature with the varying volume of a liquid, such as mercury, encased in a narrow glass tube. The liquid expands when it gets warmer and contracts when it cools down, rising and falling in the tube. Markings on the tube show the temperature.

Liquid thermometers are still used today for some purposes, but there are now other kinds of thermometers available, including ones that operate electronically. These instruments work by detecting variations in the flow of electricity through the thermometer’s circuitry caused by changes in temperature.