A pocket gag
The spectator selects a card, and the magician states that he has a matching card in his pocket. As he begins to remove the card, it appears to be the three of hearts. The spectator says that the chosen card was not the three but the eight of hearts. The magician continues to remove the card, which is longer than a normal card.
When the card is removed from the pocket, the spectator can see that it is the eight of hearts.