If you haven't played Sudoku, you probably have at least one friend who has earnestly tried to convince you that it's one of the best ways to waste your time. And if you just have a second, they'd be happy to show you right now exactly how it works. Sudoku converts are a rabid bunch.
But there's a reason for it. Sudoku is pretty easy to set up: You have a 9x9 grid, and within the grid are nine 3x3 squares. The grid is sparsely populated with a few numbers. Your goal is to make each column and row contain the numbers 1 through 9 -- no repeats. Beyond that, you must make sure each 3 by 3 box contains the numbers 1 through 9.
A Sudoku rookie might be surprised, then, to learn there's actually no math involved at all -- which means that even children can become obsessive about filling little squares with numbers. And for those kiddos who cannot abide by the pencil-and-paper version available in many books or daily newspapers (or perhaps don't recognize a hard copy of a newspaper), there are tons of online versions to play.