how thermometer works?

A thermometer is a device that measures the temperature in a reliable manner.
It is made of a long, narrow, uniform glass tube with a bulb at one end.
The tube appears as a continuous silver line because it is filled with mercury. (There are some thermometers which are filled with coloured alchohol too.)
Mercury is a metal which is in liquid state at room temperature. It expands or contracts with an increase or decrease in temperature. When the thermometer is filled in a warmer environment, the liquid in the bulb heats up. Due to this it increases in volume and rises in the measuring capillary tube. As the liquid reaches thesame temperature as its environment it stops rising. The number at the top of the column ofliquid in the tube gives the temperature of the body to be measured.