Let's explain how a thermometer measures temperature.
A thermometer can be said to be an instrument which is used to measure the temperature of a system.
How a thermometer works:
The thermometer measures temperature by using a mercury made glass tube which expands or contracts with respect to the temperature. At the bottom of the tube, there is a buld which holds a liquid (either alcohol or mercury).
As the temeperature rises, the liquid in the bulb expands which pushes up into the tube, while as the temperature decreases, the liquid contracts which then moves down the tube.