Are you looking to build a cool home science experiement? Why not try out a Stirling engine? Wait, what IS a stirling engine? Well... a Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work. This project was made from parts around the house and you should be able to do the same. This is a great project to explain how gears work, and how heat can be transferred into energy.