At the end of this class http://ocw.mit.edu/courses/physics/...l-mechanics-fall-1999/video-lectures/embed21/, Prof. Walter Lewin spins a plastic object on the table. The object starts spinning, it stops and immediately starts spinning in the opposite direction. The question is why? Why doesn't it just stop? The question is left unanswered and I could not come up with any reasonable idea. Can anyone give me a hint? I'm really puzzled by this! Thanks!