For the benefit of those who may not know, Sudoku is a logical puzzle game. Many people are put off by seeing a grid with numbers in it, but in fact the puzzle doesn't require any arithmetic at all - just deduction and logic.
The rules of game are that you should fill a number in to every cell in the grid, using the numbers 1 to 9. The restriction is that you can only use each number once in each row, each column, and in each of the 3x3 boxes.
For e.g., in the puzzle above, the three red questionmarks are in places where there is a missing value - but it is easy to see which number to place in each. If the rest of the line or box is complete, then by a process of elimination, you know which value must be left!
In this case, the only number missing from the horizontal row is a 3.
The number missing from the vertical column is a 9.
The value missing from the box is a 4!
Very addictive indeed