Question

on what principle are the thermometer based .
why do most thermometer use liquid?

Answer 1

Thermometers are based on the principle of expansion in liquids.
When temp increases mercury inside the thermometer expands to give reading of temperature

Answer 2

Thermometers are based on the property of expansion of liquid due to heat. Most substances belonging to the main 3 states of matter.. Solid, liquid and gas show heat expansion. Commonly used clinical thermometer use mercury as its melting point and fluidity, viscosity etc. properties are suitable to measure body temperature.
In labs, along with mercury ALCOHOL thermometers are also used. The alcohol is colored red for ease in taking readings.