After reading the post about the two magicians I thought I would post a card trick that my dad taught me a long time ago and I never quite figured out how it works. I am guessing it has to do with modular arithmetic. I am sure some of you have done this trick before.

1) Take 21 random cards from a deck

2) Have a person choose a card and put it back randomly

3) Arrange the cards into 3 columns of 7 with the first card going as the first card of the first column, second card as the first card of the second column, third as the first card of the third column, fourth as the second card of the first column and so on.

4) Have the person pick which column their card is in

5) Put the columns into piles and put the chosen column in the middle of the other two

6) Repeat 3-6 two more times

7) Their card will be the 11th card in the deck, works every time

I've been doing variations on this trick since I've been like five and I haven't thought about it enough to figure out how it works. If my explanation doesn't make sense, try this one on the internet: http://www.wikihow.com/Do-a-21-Card-Card-Trick