The Speculist

So that I'm not the last geek to get addicted to solving Sudoku puzzles, click here.

You'll start with a partially filled grid like this:

And then you fill it. There's just one rule:

Fill in the grid so that every row, every column, and every 3x3 box contains the digits 1 through 9.

Deceptively simple. Wikipedia has a lengthly article on the game.

Published puzzles often are ranked in terms of difficulty. Surprisingly, the number of givens does not always reflect a puzzle's difficulty. A puzzle with a minimum number of givens may be very easy to solve, and a puzzle with more than the average number can still be extremely difficult. The difficulty of a puzzle is based on the relevance and the positioning of the given numbers rather than their quantity.

Computer solvers can estimate the difficulty for a human to find the solution, based on the complexity of the solving techniques required. This estimation allows publishers to tailor their Sudoku puzzles to audiences of varied solving experience.