To make a heat detector.
Stretch a rubber band around the sides of a wooden box. Then cut out a arrow from the piece of cardboard and, mount it on a pin, and push the pin under the rubber band. If you now bring a burning match or candle close to the rubber band, the cardboard arrow will rotate anti-clockwise. Move the flame near the other end and the arrow rotates clockwise. The heat causes a portion of the band to contract, rotating the pin and arrow. This is because rubber contracts when heated and expands when cooled.