Bi-metal strip
Each metal expands at a different rate. When two metal sheets are fused together and heated up, because one of the metals expands at a faster rate, the two strips will begin to curl to make up for the difference in expansion.
Thermodynamics
Ball and ring
Initially the ball is just shy of being able to fit into the ring. But if the ring is heated up, it will expand slightly so that the ball can finally fit.
Cloud formation
This demonstration shows the process by which clouds form by rapidly reducing the pressure inside a bottle with alcohol vapor inside, as the pressure decreases the alcohol rapidly condenses to form a cloud.