The jumping disc toy works on the same principle as a thermostat or a shunt-valve in your car. With such a thermostat or automatic valve, a special piece of metal, called bi-metal, moves when heat is applied or removed. Bi-metal is actually two pieces of thin metal fused together in the middle. When the temperature changes, the metals expand differently and bend into a curve in either direction.

 For instance, when the temperature drops in your home the bi-metal in your thermostat cools down, moves a little, hits a contact and turns on a heater. This way the temperature stays the same. In a car, the metal disc made of bi-metal opens or closes the valve to the radiator, so the water stays about the same temperature to make the engine run well. 

In the case of the jumping disc, the bi-metal stays in one position when at room temperature, and in another position when at body temperature. First you warm it to body temperature in your hand, and then you snap the disc between your fingers so it stays in this “new” position. Now you carefully, and quickly, slide it onto a hard surface, like a table. In a few seconds the disc cools off, suddenly snaps back into the regular position, and with a loud “click” flies high into the air!