Rock Throwing

The problem with rock throwing has to do with velocity. The speed of a thrown rock is slow enough you can see it's approximate destination before it arrives and still have time to change your mind and wish the rock hadn't been thrown. The problem is, you can't unthrow a rock. All that is left to do is to holler "Duck!", or turn and point disgustingly at someone standing near you, preferably not one of your few friends.