Talking with a friend about the perpetual motion (we both know it don't exist but don't know how to prove it) I came with the following system. Say you have an almost empty magnet sphere. I say almost because at its center it has a little magnet such that it is repulsed by the internal walls of the sphere. If we shake the system a little bit I guess the magnet at the center of the sphere will start to oscillate. Ah I forgot to precise that there's no air into the sphere so that there is not friction in the motion of the little magnet at the sphere's center. If I didn't know that perpetual motion don't exist, I'd say the little magnet would oscillate forever. Clearly it has a potential energy and as there's no perpetual motion, this energy must convert itself into another form of energy. In this case, what would it be? (I'm pretty sure the sphere would gain energy like heat due the electronic stuff... but I didn't study this part of Physics yet so I'm just speculating).