Perpetual motion machines are devices that are claimed to operate indefinitely without an external energy source. They are impossible to build because they would violate the laws of thermodynamics. One of the most common ways that people try to build perpetual motion machines is by using magnets repelling each other. However, this is not possible because magnets do not repel each other indefinitely. The force between two magnets decreases as the distance between them increases. Eventually, the force will become too weak to overcome the friction of the machine, and it will stop moving.