Card-guessing tricks give a magician the opportunity to show off his or her mind-reading prowess. In many cases, the illusion of mind reading arises not from sleight of hand but as a consequence of some mathematical principle.
One of the most startling of such prediction tricks is known as the Kruskal count, named for Rutgers mathematician Martin D. Kruskal.
The magician invites a subject to shuffle a standard deck of 52 playing cards, then secretly pick a number between 1 and 10. The subject then slowly and steadily deals out the cards one by one, face up, to form a pile. As she does so, she goes through the following counting routine: Suppose her secret number is 6. The sixth card that she deals becomes a “key” card, and its face value tells her how many more cards must be dealt to get to the next key card. For example, if the key card happens to be 7, she starts counting again, this time from 1 up to 7, to find the next key card. She repeats the procedure until all 52 cards are dealt. An ace counts as 1, and a king, queen, or jack count as 5.