Infoseek trick

A plastic card is shown to have a crystal ball printed on it. The Magician turns his back and asks the spectator to think of a number between 1 and 52 then writes this on the crystal ball, once this is done the card is placed face down. The magician truns around and places the face down card into a small envelope and then into a small leather holder making it impossible for the thought of number to be seen, Now the Magician introduces a deck of cards and before taking them out of the case the Magician names 1 card that is in his mind.The spectator is now asked to name the number on the crystal ball and then count down into the deck turning the cards face up as they come off the top....to thier astonishment the card at their number matches the Magicians named prediction!!!