Thermometers are special devices that are used to determine temperature that is, how hot or how cold something is!
One of the most common kinds of thermometers looks like a long, enclosed, clear tube with colored liquid inside (often red). The type of liquid inside the tube naturally expands and gets bigger as it gets warmer and gets smaller as it cools down. This causes the liquid to climb up the thermometer when it detects heat, and to drop down when it detects cold. The outside of the thermometer has a scale on it, which tells you what temperature the liquid has detected!
Lots of people now also use digital thermometers. Instead measuring temperature by how much a liquid does or does not expand, most digital thermometers use sensors that detect heat. Some of these measure from inside your ear or under your tongue to tell you the temperature of your body!
The most common temperature measurements are degrees of Fahrenheit and Celsius. These scales give numbers to certain temperatures based on how cold it must be for water to freeze, and how hot it must be for water to boil. The scales assign certain degrees to these measurements so that we can use them to talk about freezing, boiling, and everything in between!