WOW

Magician asks a spectator to draw a card freely(no force), and sign it.
Then put the card back into the deck, and shuffles the cards.
The magician shows one card on the top of the deck,but it is not the same card the spectator signed.
Put the card into a clear plastic sleeve,the spectators can see the face of the card is still not the signed card.
The magician holds the sleeve, then the card slowly and visibly changes into the signed one!!
The card can be removed from the sleeve by spectator and there is nothing else in the sleeve!!
Anytime when spectators saw this trick,they would sound the same word:WOW!!