Son and I were looking through Toys-R-Us after Christmas, searching for a way to spend a gift card he got, and we passed through a book section. Sitting on a box, out of place, was the book "Sudoku for Dummies". I like the Dummies books, (although I tend to not want to tell the world via my bookshelf that I'm a dummy; let 'em figure it out on their own), and usually, they only put out Dummies books on popular topics, so I picked it up.
And was instantly doomed.
Back in middle school, I got some logic word puzzles, where by elimination you filled in a grid, searching for, i.e. each person's hair color, eye color, pet's name, etc., given clues like "Ben doesn't have green eyes", etc. I loved 'em. The easy ones only had three or four sets of clues to resolve, while the harder ones had more like 9. I went through them all.
Enter Sudoku. It's similar, except no word puzzles. It's just a nine-by-nine grid, with a few numbers already filled in. Trick is, each number 1-9 can only appear once in each row, column, and 3x3 cell. And that's all the clues you need to fill in the whole puzzle, number by number.
They come in various levels of difficulty. Son has been enjoying the easy ones, while I've been doing the harder ones, and am currently trying some that are 16x16, rather than 9x9.
If you're curious, there are sites that describe the game much better, and others that will generate you games to play, for free.
But be warned. It's addictive!