Two small electric motors are connected to a device which takes a thermal difference and converts it into a voltage. One engine is placed into resevoirs of boiling water and ice water which turns the propeller. The second engine is then placed into the ice water and a resevoir of liquid nitrogen and its propeller starts to turn. Demonstrates that work depends on resevoirs of different temperature. Students are usually surprised to see work come out of a resevoir of ice water.