The Sudoku puzzle must be well known to everyone by now and its rules are simple. Try building a 9x9 Sudoku puzzle with suitable formulae and formatting for the cells (remember this page is about the SUM function).

One of the rules for Sudoku is that you must use each digit in the range 1-9 once only. This would be difficult to implement in Excel (needs VBA programming) and you should not attempt it here.

You can check that each row and column add up to the correct sum (1+2+3...+9) and take some action if they do. This does not guarantee that the player has used all nine digits as required but it tells the player that he has the correct total and a quick scan of the line or square will tell him if he has used any digit more than once.

One possible course of action is to format the total squares in a noticeable way using conditional formatting. Select the cell you want to format and choose Conditional Formatting. Choose 'equal to', add the target value and create the format when the value is achieved. This will give a visual clue when the player has matched the game requirement for a row, column or block. You can apply conditional formatting to other cells using the format painter so there is no need to repeat the action a further 26 times.

The Sudoku solution in Excel creates a simple mathematical model: the player enters data (the inputs, numbers 1-9), Excel performs a calculation (process, adds them up and checks whether they add up to a certain value) and displays the results (output, the sums of the rows, columns and blocks).

Another issue for Sudoku is the creation of new games. You might like to think about how you would do this for a 3x3 grid, placing a number in the range 1-9 in a small number of squares. A little thought should convince you that you need some programming knowledge for this, either in Excel VBA or a language such as Delphi. This takes us well away from the SUM function.