Sudoku

A sudoku puzzle is a grid of nine by nine squares or cells, that has been subdivided into nine subgrids or "regions" of three by three cells.

The objective of sudoku is to enter a digit from 1 through 9 in each cell, in such a way that:

Each horizontal row contains each digit exactly once
Each vertical column contains each digit exactly once
Each subgrid or region contains each digit exactly once

This explains the name of the game; in Japanese, sudoku means something like "numbers singly".
Solving a sudoku puzzle does not require knowledge of mathematics; simple logic suffices. (Instead of digits, other symbols can be used, e.g. letters, as long as there are nine different symbols.)

In each sudoku puzzle, several digits have already been entered (the "givens"); these may not be changed.

The puzzler's job is to fill the remainder of the grid with digits –respecting, of course, the three constraints mentioned earlier.

A "good" sudoku puzzle has only one solution.

In spite of the game's apparent simplicity, sudoku can be highly addictive.