By Emily Mitchell/Reported by Wendy Cole

How many shuffles will thoroughly mix a deck of cards? The world wasn't exactly waiting for an answer, but Columbia University mathematician Dave Bayer wanted one anyway. He and Harvard mathematician Persi Diaconis observed thousands of shuffles and used several computers. After analyzing a deck's possible permutations (1068), they found the magic number: exactly seven shuffles ensure true randomness. A veteran poker hand, Bayer, 34, won't use his knowledge at the gaming table because "my job is to explain the problem. Others can try to take advantage of it."