Question

explain how a liquid in a thermometer change so that it can measure temperature

Answer 1

Liquid present in thermometer is called Merury.We should Use digital thermometer not clinical thermometer which is present with Mercury because Mercury is bad for Health. As a liquid warms, it expands and as it cools, it contracts. It is this action that allows liquid filled thermometers to work. The liquid is only acting according to the laws of physics. It is the scale used along side the liquid that relates to a temperature. If you are in the U.S, the it is the Fahrenheit scale that measures the freezing point of water at 32 degrees and the boiling point at 212 degrees. In most other parts of the world, the Celsius (aka centigrade) scale is used. It measures the freezing point of water at Zero and boiling at 100. The rest of the scale is just evenly divided to make the degrees. 
So, in a nut shell, the scale on the side of the thermometer measures the changes in the liquid volume as affected by temperature. The rest is thermodynamics.