We were watching an old episode of Universe which talked about the creation of craters from the impact of meteorites on the moon. In one of them, there was clearly a hill at the centre of the impact crater which seemed odd to me. When I said that out loud, DS explained, "Newton's Third Law, you know. For every action, there is an equal and opposite reaction." I'm not sure if that's correct, but it made a lot of intuitive sense to me.