You're cruising down the street on your skateboard. In one hand you have a tennis ball. Halfway down the street is an X painted on the ground. Your job, should you choose to accept it, is to drop the tennis ball from your hand so that it hits the X. When do you drop the ball?

If you try this experiment yourself, you will find that most people will wait until they are right over the top of the X before letting go of the ball. This, however, is incorrect. If that was your first guess, think again. Why won't it work?

Because you are carrying the ball, it already has a forward momentum. Drop it right over the X and it will land somewhere past where you want it to be. The effect will be the same as if you had stood over the X and thrown the ball forward. To hit the X you need to drop the ball before you have reached it. And the faster you are moving, the earlier you need to let go.

This is a difficult thing for a lot of people to believe. In one survey done on undergraduate students, about half of them argued that the ball can't have any forward momentum if it's just passively carried. True, the carried ball feels like it is sitting still, but don't be fooled. From the point of view of someone standing by the X, the ball is moving as fast as you are.