Here's why:

* Coefficient of Thermal Expansion: This is a material property that describes how much a substance expands or contracts in response to temperature changes.

* Bimetallic Strip: This is a strip made of two different metals with different coefficients of thermal expansion, bonded together.

* Cooling: When cooled, the metal with the higher coefficient of thermal expansion will contract more than the metal with the lower coefficient. This difference in contraction causes the strip to bend towards the metal that contracted less (the metal with the lower coefficient of thermal expansion).

Example:

* Brass has a higher coefficient of thermal expansion than steel.

* When a brass-steel bimetallic strip is cooled, the brass will contract more than the steel.

* The strip will bend towards the steel side.

Note: The opposite happens when the bimetallic strip is heated. The strip will bend towards the metal with the higher coefficient of thermal expansion.