### Theory:

Units of Temperature:
There are three units commonly used to measure the temperature:
1. Degree Celsius
2. Fahrenheit and
3. Kelvin.
Degree Celsius:
Celsius is written as $$°C$$ and read as a degree. For example, $$100$$$$°C$$ is read as hundred degree Celsius. Celsius is also called a Centigrade. Temperature Measurement - Degree Celsius

Fahrenheit:
Fahrenheit is written as $$°F$$. For example, $$100$$$$°F$$ is read as hundred degree Fahrenheit. Temperature Measurement - Fahrenheit

Kelvin:
Kelvin is written as K. For example, $$100$$$$K$$ is read as hundred Kelvin. SI unit of temperature is Kelvin. Temperature Measurement - Kelvin

Measuring Temperature:
The temperature of the substance is the approximation of the average kinetic energy of the substance. Higher temperature means the molecules inside the substance are moving rapidly. Similarly, lower temperature means that the molecules are moving in a slow phase.

But the question here is, how to measure the temperature?

The movement of the molecules is directly proportional to the temperature. Since the molecules in any substance are very small to compute, it is really difficult to measure the movement of the molecules (Kinetic energy). So we need to look at indirect ways to measure this movement (Kinetic energy) of the molecules.

Solids, liquids and gases are affected by heat. When we heat, the substances start to expand, and they start to contract when we cool them. This principle is used to measure the temperature. The thermometer is used to measure the temperature.
In a thermometer, when liquid gets heated, it expands, and when it is cooled down, it contracts. This principle of expansion and contraction is used to measure temperature.
Reference:
https://pixabay.com/fr/vectors/thermom%C3%A8tre-temp%C3%A9rature-1134182/