The Jack Puzzle
Effect: A simple guessing game involving two jacks and an ace is proposed. The spectator gets the ace while the performer holds the two jacks. One of the jacks is openly exchanged with the spectator's ace. When the spectator tries to guess which jack she has, it's discovered that she still has the ace. To make matters less confusing, the ace is placed aside - leaving just the two jacks. One of the jacks is handed to the spectator. She's asked to guess which jack she holds and once again the card in her hand has changed into the ace. The performer admits that he rigged the game by using one of those special "trick invisible jack" cards. The spectator places her hand over the "invisible jack" while the performer hangs on to the ace. When the spectator lifts her hand she finds the performer's ace. The performer's card is turned face up and revealed to have changed places with the invisible card — having transformed itself into a genuine jack.