This attractive and economical Stirling Engine is both engaging and a great educational tool. It runs by exploiting the heat flow generated by the small temperature differences between room temperature and a container of hot water or ice. The Stirling Engine is an External Combustion Engine (the heat source is outside the engine) and can use any available source of adequate temperature. It is a classic illustration of converting heat energy into mechanical work using a thermodynamic cycle similar to the well-known Carnot cycle. In this case, however, the engine needs two pistons, a small power piston and a larger "displacement piston" that also stores heat energy. The engine works by transporting heat from a warm surface to a cold one, and larger temperature differences make the engine rotate faster. The model runs at about 150 rpm when powered by a cup of hot water. The direction of rotation can be changed by switching the warm and cold surfaces. It is also possible to use Stirling engines "in reverse" as a heat pump by putting in mechanical energy and creating a temperature difference. Although possible, that would be tedious for this model, but it graphically illustrates how much work goes into making heat!