Let's swap a card

Some cards are shuffled, separated in two face down piles and one pile is handed to a spectator. Both the magician and the spectator put their respective piles behind their backs, freely choose a card, and then swap them. These cards are put back in the center of the respective piles upside down.

When the two piles are fanned, the chosen cards turn out to be exactly the same! Just when the audience starts to believe that all the cards must be the same, the two piles are turned face up and the cards are shown to be the same… but all different from the two selections!