A blank su doku grid consists of nine rows and nine columns subdivided into nine 3 x 3 subgrids. Su doku has two simple rules:

• Each column, each row, and each box must contain each of the numbers 1 to 9.
• Therefore, no column, row, or box can contain two squares with the same number.

Each puzzle starts with a set of clue numbers placed on the grid. Logic is all you need to solve a su doku. No addition, subtraction, division, or multiplication is required. However, you do need to ask yourself questions like 'If so-and-so number is in this column, will such-and-such number go in this other box?' Your answer to these questions will always be either 'yes', 'no', or 'maybe'.

Use a pencil and eraser. Solving su doku, especially the more difficult puzzles, requires you to make notes of optional numbers. These notes change as the puzzle progresses, so you need to rub them out where you've solved -- or partially solved -- any numbers.