Here's why:

* Brass has a higher coefficient of thermal expansion than steel. This means that brass expands more than steel for the same temperature change.

* Steel has a lower coefficient of thermal expansion, so it expands less than brass for the same temperature change.

This difference in expansion is what makes the bimetallic strip work. When heated, the brass side expands more than the steel side, causing the strip to bend. When cooled, the brass side contracts more than the steel side, causing the strip to bend in the opposite direction.