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Sudoku is a simple but tricky puzzle in which the player aims to fill in a square grid, following only a few simple rules.

An unsolved sudoku puzzle looks like this:

 
And ends up like this:
 
The three simple rules which drive the puzzle are:

- in each column the numbers 1 to 9 each have to appear once.

 
- in each row the numbers 1 to 9 each have to appear once.
 
- in each 3x3 block (indicated by the thick black lines) the numbers 1 to 9 each have to appear once.
 
When you start a puzzle, a few squares will already have been filled in for you. From these you can deduce information about what must be in one of the blank squares - and then another, and another...