This magic trick is based on how computers detect and correct data errors. In the trick, a volunteer lays out a 5 x 5 square of identical cards colored on one side, with a random mix of sides showing. The magician adds another row and column, making sure that the extra cards make an even number of colored cards in each row and column. When the volunteer flips over a card, the row and column with the changed card will now have an odd number of colored cards, identifying the changed card. Extension activities further explore the idea of error correction, including its use in book ISBNs.