A bimetallic strip bends when heated because it is composed of two different types of metal. The different metals form the two sides of the strip expand and contract at different rates when subjected to a temperature change.
When heating a bimetallic strip, one side of the metals expands faster than the other, making it longer. Because it is bonded to the other metal, it cannot expand in a straight line. The only way the two metals can stay bonded while expanding at different rates, is for strip to bend, one side becoming longer than the other.