I'm trying to explain to my friend why the following would not work, but embarrassingly, I'm having trouble. You have a ramp that allows a ball bearing to roll up, and a strong magnet at the top which is able to pull the ball up the ramp. But there is a hole halfway up the ramp, and so the ball falls through and goes back to the start. Obviously this wouldn't really be a perpetual motion machine, but I can't explain why not. I can say thinks like "friction" or "magnets run out", but I don't feel like they are very rigorous or satisfactory answers. Thanks in advance!