Effect:

The magician shuffles the deck and then runs through the cards and asks the spectator to think of any card she sees. Let's say it's the Seven of Diamonds. The card is then lost in the deck. The magician asks the spectator to concentrate on the card. He then runs through the pack and removes five cards. He asks the spectator if the selection is among the five. The spectator confirms that it is. Through a simple elimination procedure, the magician narrows it down to one card: the Seven of Diamonds. He then says, “I'm glad you picked the Seven of Diamonds. Because it's the one card that I marked!” The magician turns the card over to reveal a large X on its back!