A heat engine is like a machine that can turn heat into useful work. Just like how a water wheel uses flowing water to turn a mill and do some work, a heat engine uses heat to make things move and do work. It has some parts inside that can take in heat and use it to make things move, like a piston that goes up and down, or blades that spin around. The heat engine needs to have a source of heat, like a fire or a hot object, and a way to get rid of some of the unused heat, like a cooling system or a stream of water.