Use any full deck and its case. Allow a friend to shuffle the cards. Tell him/her to keep one of the cards, look at it, and give the rest back to you. Then you place the rest of the cards in the case, but leave the flap open. Ask the friend to put the card back inside of the deck. The secret to this trick is holding the case. You prevent the card from going in all the way: tighten your grip on the case, while the friend is trying to put the card in the deck. (Don't grip it too hard or the card will not be able to go in at all.) Turn your back and pull out the card that is different from the rest. It should be sticking out very slightly. Take it out and reveal the card. If you pull the deck halfway out of the case it will be easier for you to know which card is theirs.