How many times should you shuffle a deck of cards? In 1992 two mathematicians, Dave Bayer and Persi Diaconis, worked it out for the ordinary riffle shuffle, where you split the deck in two and let the halves fall together: about seven. With fewer, the deck still carries its old order in long runs of cards that have only been interleaved, never really separated, and a careful player or a computer can find them. Past seven, more shuffling hardly helps. What I like is how sudden it is. The old order doesn’t fade evenly; it holds on almost untouched for four or five shuffles and then falls away over the next two or three. Diaconis had left home at fourteen to travel with a magician, which may be why he thought to ask. Most people shuffle three or four times, so most hands are dealt from a deck that still remembers the last game.