Borrow a quarter. Stick a round white label on both sides of the coin. The spectator signs one of the stickers; you sign the other. Place the doubly marked coin into a very thin brass box and cover it with a lid. The lid has a hole bored through it, so the marked coin is visible at all times.Remove the lid and allow the box to drop into the spectator's waiting hand. The marked coin has vanished in mid-air. Show the little brass box absolutely empty; your hands are empty as well.