How can you use math to astonish your friends and family? Try the 21-card trick. Filmed for Numberphile, UC Davis Department of Mathematics Postdoc Anastasia Chavez demonstrates step-by-step how this trick works with the help of some simple math.

And if you like that one, try this slightly more complex 27 card variation, as demonstrated by mathematician Matt Parker:

Watch more Numberphile and more with Matt Parker including an unexpected way to inflate a balloon, the Fold & Cut Theorem, and The Brick Double-Domino Effect Explained.

Plus: How to multiply numbers by drawing lines and Universal Calendar Puzzle: Figure out the day of the week for any date ever.

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