Thing: The Stirling Engine
The Engine itself, pictured here, does very little. Left alone above a hot surface (e.g. a radiator or a cup of tea), it simply pistons up and down, round and round until the surface cools down.
The beauty of the Engine is in what it represents. The revolutions of the brass flywheel are driven solely by the difference in temperature between 2 metal plates, and the natural cooling-heating cycle of a small space of constantly-disrupted air.
Energy is given off as heat all around us, all the time. I find the Stirling Engine fascinating purely as a visual reminder that energy doesn't always have to come from a faraway country or a huge company. It's right under our noses.