The words "perpetual motion" by themselves just mean motion that goes on forever. But usually when we say perpetual motion we are referring to a very special thing.
For hundreds of years, men have had the dream of creating a machine that, once it is set in motion Would go on doing useful work without drawing on any external source of energy. Every machine now known has to have a source of energy.

A perpetual motion machine, however, would create its own energy in the form of motion. Every time a complete cycle of its operation was finished, it would give forth more energy than it had absorbed.

Most of the people who tried to create perpetual motion machines had practical purposes in mind. They thought it would be wonderful to have machines that could raise water or grind corn without the need of supplying any energy to the machine.

Is it possible to create a perpetual motion machine? Any scientist will tell you that the answer is no. The reason is based on what is one of the most important laws of science, the principle of the conservation of energy. According to this principle, energy cannot be created and cannot be destroyed in nature. Energy can be transferred from one place to another, energy can be freed or unlocked, but energy cannot be created. This means that any machine that does work must have a source of energy.

In the course of history, thousands of attempts have been made to create perpetual motion machines. The first attempts were made at a time when the law of the conservation of energy was still unknown. A great many others were simply fakes that were later exposed.