a card trick
A subject is given a deck of cards
and allowed to choose any five; he hands the five cards to a magician.
The magician sets one of the cards aside, and asks the subject to enter
the four remaining cards into the boxes on this web page (or, preferably, on
a similar page without
this explanation). The subject then clicks on "guess fifth card".
This page is worthless without a magician who knows how to operate the trick. It would be ruining the fun if I just gave the secret away here; it's not that hard to figure out if you're so inclined.
Thanks to Robert Orenstein for the original "pick five cards" page (from which the above graphic was taken) that introduced me to this trick. Thanks also to Brian Pickrell and Bob Vesterman, who discussed this trick in rec.puzzles back in 1994.