It’s back to school after a long weekend (and the start of school for many, like our resident now-kindergartener!) so here’s a card trick that is easy enough to do while you’re occupied with either the fading summer or the burgeoning fall. (Plus if, like yours truly, you only get Right versus Left correct about 50% of the time, it’s still OK.) I’ll post a fancier card-trick tomorrow.

For this trick, start with a deck with any number of cards in it, as long as that number is 1 (boring), 4, 9, 16 (good for teaching to kids), 25, 36, or 49 (fancy, but it takes a long time to lay out so you might be back to boring). Show the cards to a Volunteer, and tell the Volunteer to **choose one** of them secretly and memorize it. Then give the deck to said Volunteer to shuffle as many times as desired and to lay out face-up in a square.

Have the Volunteer point to the row that the **Chosen card** is in. You then memorize the card on far LEFT of the row. We’ll call this card the **Identifier**.

Now gather the cards into a pile as follows: start with the card at the bottom left, and scoop up that column of cards (so the card at the bottom left is at the top of the pile). Move (right) to the next column and scoop up bottom to top. Continue until you have one pile of cards. [Alternately, you could scoop down top to bottom. Just be consistent and do the same thing for each column.]

Now lay the cards into a square row by row. [You might notice that the cards that made up the left-hand column are now the top row. Indeed, if you scooped top to bottom then you’ll just have a transposition of the original layout; scooping bottom to top masks that just a little bit.] The car you memorized in the previous square, the **Identifier**, will be somewhere in that top row. Have the Volunteer point again to the row that the **Chosen card** is in. The **Chosen card** will be in the same column as the **Identifier**, making it easy to determine. [If you lay out the rows bottom to top or a mixture of both — laying out a row and then doing rows above and below in any order — the **Identifier** won’t be at the top, but still will be at in the same column as **Chosen card**.]

At this point you could simply state what the **Chosen card** is, or you could get all fancy and just memorize it, then gather the cards up, look at them, think about it, and then announce the **Chosen card**. Hooray!

*This trick is a variation of one found at card-trick.com.*

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This entry was posted on September 3, 2008 at 6:57 pm and is filed under Math Magic. You can follow any responses to this entry through the RSS 2.0 feed.
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