Why does a thermometer that uses a liquid to measure temperature work?
Transfer of energy from one source to another.
As heat is introduced into a system, the molecules absorb the thermal energy and increase their kinetic energy. The measurement of temperature is the resultant impact of the exchange of kinetic energy between the system and the molecules or atoms within measuring instrument (ie. a thermometer using either mercury - atoms or alcohol - molecules ) or increase in thermal energy from a base line using the infra-red spectrum.
Hence the measurement of temperature is the measurement of the average kinetic energy within that system.
The system can be any thing from outdoors or car running temperature or anything else that one may wish to measure the temperature of.