Lorayne's To-And-Fro magic square reminded me of a little puzzle that people find VERY tough to solve, yet with a little trick you can solve it very quickly. It's an impressive mental feat.
It's related to the magic square in this trick, but with an additional level of toughness.
Rather than a "magic square" theme, it's a card puzzle that is an interesting mental demonstration.
You can also make a tie to the Sudoku puzzles which are currently the rage.
I'll describe the puzzle here, and in a follow up post give tips on the method. Don't read the next post until you've tried it without any help - to get a good understanding of how tough this is to laymen.
Take out the all four of the Aces, Kings, Queens, and Jacks. Shuffle them, and deal in a 4 by 4 square.
You solve the puzzle by switching pairs of cards so that each row and each column contain an ace, king, queen, and jack, as well as one of each of the 4 suits.
Have at it.