Explain the physics behind a thermostat that functions using a bimetallic strip.

Think about volume expansion, in terms of length, brought about by increased temperature.
Different solids have different thermal expansion rates. It is that difference that is the basis
of bimetallic strips.

Multiply the 3 dimensions, assuming it is a rectangular cuboid, to get the volume in cm³ divide mass by volume to get density in g/cm³