Used in conjunction with a heavy object (perhaps Janice the bowling ball), it can demonstrate conservation of momentum. Have someone (perhaps an unruly student) stand on the skateboard and then chuck the bowling ball at him/her while yelling, "Catch!" If done properly, the student-skateboard-bowling ball system will move together. Alternatively, have someone (perhaps someone who is rather trustworthy) stand on the skateboard and throw the heavy object at you, which should cause the person and skateboard to move in the opposite direction from the ball.