## Posts Tagged ‘card tricks’

### Mind-Reading and Card Tricks: the Kruskal count

December 4, 2007

Want another mind-reading trick? This one uses an ordinary deck of cards and is very impressive, although it only works about 85% of the time.

Here’s how to play. Have a person think of a number n between 1 and 10 and keep that number secret. You shuffle a deck of cards, and then slowly deal the deck face up card by card as the person watches. The nth card is a “key” card, in the words of Ivars Peterson, and the value of that card — counting aces as 1, face cards as 5, and all other cards as the number shown — tells the person how many cards ahead the next key card is. For example if the initial number is 7, then the 7th card is the first key card. If that card is a Jack (worth 5) then the person counts ahead (as you continue to slowly deal) 5 more cards, and that 5th card is the new key card. If that card is a 3, then the 3rd card after that is the next key card, etc. All of this is done in silence.

Eventually the deck runs out. You look at the person and, using your amazing mind-reading/mathematical powers, you show them their final key card! Click here to find out how it’s done.