This hands-on activity clearly illustrates how a thermostat works by demonstrating differential expansion. Students heat the bimetallic strip in the model and watch it bend to make contact, complete the circuit and light the lightbulb. After the strip cools it bends back, breaks the contact, and turns the light off. When the bimetallic strip is heated and cooled, the dimensions of the two metals (brass and steel) expand differently over their long dimension. Since the two metals are fused together, one side bends more than the other causing the overall bending of the strip. A small lightbulb, socket, wire and dry cell are needed to complete the circuit and are sold separately.