The puzzle sensation that's sweeping the nation.

Sudoku. They're everywhere.

Maybe you've heard people talk about them, or seen them in the newspaper, or watched them pop up on the bestseller lists. The puzzles appear to have originated in the United States in the 1970s, but spent a couple of decades in Japan (where they got their name) before migrating back to the Western Hemisphere again. It's no wonder they've caught on: they require no math and are easy to learn how to play. Just be careful you don't get addicted — like I have.

The best web site I've found for playing Sudoku is websudoku.com. With four levels of play, and options that keep track of your best time and allow you to "pencil in" guesses, it's very flexible and, even better, very free.

If you're interested in more history of Sudoku, or want to read up on strategy or the theory that makes sites like WebSudoku work, the Wikipedia article on the subject is pretty good, as is this column from the website of the Mathematics Association of America. (I know, I said they require no math, and I promise they don't. Mathematicians just seem to be fascinated by them as much as everyone else.)

And, of course, if you like your Sudoku-training in print, we've got Su Doku for Dummies.