This magic trick with cards consists in that, with 21 cards, you choose any one and without the performer of the trick seeing it, you can guess it.

The process is the following:
A card is chosen, the deck is shuffled. When the shuffling is finished, deal the cards in order into 3 groups of 7 cards. This procedure is done 3 times and each of those times the spectator who knows his card has to tell you which deck it is in of the three. You always have to put the deck of the real card in between the other two leftovers. After repeating this process 3 times, you do it a fourth time but while you do it the card number 11 will be the correct one chosen by the spectator. It is a mathematical probability after all.