21 cards trick is a math card tricks. Using math principle to identify which one is the chosen card among 21 cards.
21 random cards deal in 3 column with 7 cards in each rows. The spectator choose any card they wants. The performer only need to know in which column the card belong to. After that the performer took all 21 cards and make into one pile and then deal the cards again in the same way (3 x 7 cards) as before. Then the spectator being asked again, in which column the chosen card belong to. After that the cards been collected and re-deal in the same way as before, and the spectator being asked the same question.
After only 3 times asking the same question, the performer already can identified which one is the chosen card among 21 cards.