Here's how they work:

* Composition: A bimetallic strip is made of two different metals with different thermal expansion rates. This means one metal expands and contracts more than the other when heated or cooled.

* How it works: When the temperature changes, the strip bends due to the difference in expansion. This bending action is used to:

* Open or close an electrical circuit: In older thermostats, the bending strip would physically open or close a contact point, controlling the heating or cooling system.

* Adjust a sensing device: In newer thermostats, the bending strip might move a sensor, which then sends a signal to the HVAC system.

In summary: The two metal pieces in a thermostat are bimetallic strips that utilize the principle of different thermal expansion rates to control the temperature.