Marble Stirling engine
Marble Stirling Engines are simple to make and fun to watch. It will teach you a lot of thermodynamic concepts. source: youtube user Ludic Science
This is a marble-displacer Stirling engine. The rolling marbles act as the displacer, which are transferred from the hot end to the cold end by the tipping of the test tube. The tipping of the test tube is caused by the piston being connected directly and rigidly to the crank disk and flywheel. source:youtube user 1967geezer