A half-dollar is borrowed and marked. It is placed into a wooden coin slide for safekeeping. Next, show a small box with two doors in the front (looks like a miniature sliding die box). The box is absolutely empty. Remove the marked half-dollar from the little slide and place it into the box. Tip the box one way, open one door and show the box empty. "It's on the other side." Tip the box back, open the other door and show it empty. "It's on the other side." Back and forth. They can hear (and even see) the coin sliding. Yet you will only open one door at a time. Finally, you open both doors, and the coin has vanished. But where could it be?? Ask the spectator to pick up the coin slide and see what's inside. Sure enough, it is the marked coin.