I found these stones on the beach. They look like normal stones but they're not.
This one I found is called a 'celt' or a wobblestone or a rattleback. It is a semi-ellipsoidal stone which will rotate on its axis in a preferred direction. If spun in the opposite direction, it becomes unstable, "rattles" to a stop and reverses its spin to the preferred direction. This spin-reversal appears to violate the law of the conservation of angular momentum.
Next time you go to the beach, take a bread board or a piece of plywood maybe and see if you can discover one or two canoe shaped stones that can spin on a flat surface and then magically spin in the opposite direction.
