unstoppable force?

Ok so my seven year old came up to me and asked me a question the other day. I told him he would have to ask his mother she told him to ask me. So the question is: "What happens when an unstoppable force hits an unmovable object?" Now what should I tell him I told him I would ask some friends for the answer. What should I tell him? Please help so I can give him a good educated answer.