A thermometer measures temperature using mercury which, like most things, expands when it gets hotter.
As it expands, the column of liquid mercury fills the tube, and you can read how hot it is on the scale – measured in degrees Celsius or Centigrade (°C)
There are many other types of thermometer, but they all do the same job. They tell us how hot something is.
The scale on the thermometer is based on the melting point of ice (0 degrees or 0°C) and the boiling point of water (100 degrees or 100°C).
Temperature measurement is needed for many essential things such as keeping food fresh in your fridge at home, keeping your house comfortable with central heating and cooling your car with air conditioning to make it bearable on a hot day.