Thermostat works on the principle of ...

Thermostat is a technological appliance by which it senses the ambient temperature and maintain a set point temperature. It is usually made of two metallic strips having slightly different thermal expansion coefficient. This result in a bending of the bimetallic strip that can be used to break an electrical circuit at high enough temperatures.

Hence  the thermostat works on the principle of thermal expansion of solid materials. That is unequal expansions of different metals on heating.