Top 5 Reasons to Like Mathematical Card Tricks

Effect: "All of us have had the experience of losing socks while doing laundry. Some of us lose a sock every time; it's baffling. Then we end up with all these odd socks! There is a simple solution of course, that only a mathematician would think of. As long as you have an even number of odd socks, you can define your own pairs." Take out a deck of cards, and hand out for shuffling. "Laundry services in hotels, of course, don't have this problem, they always seem to get it right. I think I've figured out their system. I'm going to lay out ten pairs of socks hereI never said they matched perfectly, in fact all of them are made up of loners left over from previous relationships." As you speak, take the deck back, and count off twenty cards. Deal out this packet in a haphazard manner into face-down pairs, occasionally flashing card faces as you proceed, perhaps commenting on the odd matchings. Then tum away, and invite up to ten audience members to each peek at a different pair, and note both cards. "It's important that you be able to recognize your own socks later, so remember what they look like." Have one person gather up the pairs in a random order and hand you the packet as you tum back. "Okay, it's time to do the laundry! It looks like I have a full load of socks today." Repeatedly cut the cards as you speak. "Round and round they go. I'm one of those people who doesn't believe in spin-drying, so I'll lay them out here to dry instead." Deal the cards out face down, one by one, in what once again seems to be a haphazard fashion, to form a rectangular array. "I have four lines of socks here, with five socks in each." Tum all of the cards face-up, and remark, "I think they are dry already." Now ask each person in tum to tell you in which line(s)/row(s) they see their socks/cards. As each person reveals their lines/rows, you pick up their socks/cards and return them.