Ask the spectator to pick a number from 1 to 10, without telling you what it is. Tell him that you are going to show him 10 cards, and he should memorize the card which is at the number that he had picked. In other words, if he picked the number 7 then he should memorize the 7th card that you show him...

Pick up the top card from the stack of 21 cards (from the previous trick) and say "one" and show it to him for a moment, then put it face-down onto the table. Show him the next card for a moment and say "two," then place it face-down on top of the first card. Show him the next card for a moment and say "three," then place it face-down on top of the stack. Keep doing this until you have shown him 10 cards.

Now pick up the stack of 10 cards that you just dealt and hold it face-down in your hand, then ask him what his number was. Move the bottom card to the top of the stack, then move the next card from the bottom to the top of the stack, etc., while you silently count up to his number. In other words, if he picked the number 7 then you will move 7 cards (one at a time) from the bottom of the stack to the top. After you have silently counted to his number while moving the cards from the bottom of the stack to the top, keep going so that you move 3 extra cards to the top.

Now give him the stack of cards, face-down, and tell him to deal one card face-down onto the table, then he should move the next card from the top of the stack to the bottom of the stack, then he should deal the next card face-down onto the stack on the table, and so on. Make sure that he perfectly alternates between putting a card onto the stack on the table and putting the next card on the bottom of the stack in his hand, and tell him to stop when he is holding the last card. When he stops, ask him what his card was and then tell him to turn over the card that he is holding. It's his card!