This trick was originally done by David Blaine. It requires a duplicate card - which means the deck can't be examined afterward - but it leaves the spectator speechless. This trick can be done with one spectator, or with slight modification it can be done with two spectators.
The magician shows the top card and asks the spectator to remember it. He then turns the card face-down again and tells the spectator to make sure that nobody can get to the card (sandwich it between hands, drink on top of it, etc). He turns over the next card and says that it will be his card, then returns it to its original face-down position. Next, he tells the spectator to concentrate on their card as he waves his card around theirs. When he is done, both the magician and the spectator turn over their cards. The cards have magically switched places!
You will need:
• A deck of playing cards
• A duplicate card from another deck
Step 1: Secret Setup
Before performing the trick, place a chosen card (I use the Ace of Clubs) second from the top of the deck. Then, place an identical card from another deck on top. The order should go like this: [Ace of Clubs], indifferent card, [Ace of Clubs], then the rest of the deck.
Step 2: Show the Spectator's Card
Perform a double lift and pick up the top two cards as if they were one card. Show the indifferent card to the spectator, explaining that it will be his card and that he needs to remember it. Turn the cards back over and hand the top card (the real top card) to the spectator and instruct him to make sure nobody could possibly get to it without someone noticing. This could include setting a drink on it or sandwiching it between his hands.
Step 3: Show Your Card
Perform another double lift and show the [Ace of Clubs], saying that it will be your card. Turn the cards face-down again and take the top card (the spectator's card).
FOR TWO SPECTATORS:
This will be the second spectator's card, not yours.
Step 4: Magical Mumbo-Jumbo
Tell the spectator to concentrate on his card and imagining it switching with your card. Make up some sort of magical mumbo-jumbo while waving your card around his. What he doesn't know is that the cards have already been switched.
FOR TWO SPECTATORS:
Simply tell the spectators to focus on each other's cards. No need for card waving.
Step 5: The Reveal
Instruct the spectator to turn over their card, and turn yours over simultaneously. They will be utterly amazed that the cards seem to have magically switched!