Sudoku

Sudoku is a popular Japanese number puzzle that I've started becoming addicted to. The basic idea is that you are given a 9x9 grid, subdivided into 9 3x3 subgrids. Each of the grid squares can hold one number between 1 and 9. Some of them are already filled out for you.



Your job is to fill in the blank squares, given the following rules:
* Each row can only contain one of each number.
* Each column can only contain one of each number.
* Each 3x3 subgrid can only contain one of each number.

Using these 3 simple rules, you can logic your way through the entire puzzle, though it's often not easy. I like to play on this this web site, because it's free (you do have to register though, which requires a valid email address), you can pick your difficulty level, it keeps track of your time for you, and you can pause it.

If you are just getting started, sudoku.com (which is not free) has a good how to solve section that explains all the strategy you'll need.