A perpetuum mobile (Latin for "forever moving") is an apparatus capable of doing work without supply of energy. The illustration below, based on a sketch of Jacopo de Strada from 1580, gives an example. The machinery is supposed to drive a grindstone. The water wheel drives the grindstone axis and an Archimedes screw, which in turn raises the water into the reservoir, from where it returns to the water wheel and keeps the machine operating forever.
The perpetuum mobile is a fallacy. It is impossible to build. Even if all friction could be eliminated the perpetuum mobile could only drive itself. The slightest amount of work extracted from it would require input of energy to keep it operating.
The perpetuum mobile has been the dream of inventors since ancient times and continues to pop up in sketches and drawings of would-be inventors every now and then.