Armando Vera's Cell-O-Phane

The magician writes and shows a prediction on a card case's cellophane - the Jack of Spades, for example. Immediately afterwards, a spectator is asked to sign the back of the cellophane and to hold the card case between his hands. Then an audience member chooses a card that is supposed to be identical to the one the magician just wrote. To everyone's surprise, the chosen card is the Nine of Hearts! The magician waves his hands on top of the card case and reveals that his prediction has changed to the Nine of Hearts, with the spectator's signature on the other side!