Here's why:

* Bimetallic Strip: Thermostats use a bimetallic strip, which is made of two different metals that expand at different rates when heated. This difference in expansion causes the strip to bend.

* Temperature Control: The bending of the strip is used to control a circuit, turning a heating or cooling system on or off based on the temperature.

Let me know if you'd like more information on how thermostats work!