The short explanation

• Click on a number once to mark it as a possible solution of that cell.
• Double-click on a number to mark it as the solution of that cell.

The detailed explanation

• Move your mousepointer over a sudoku cell to bring up the nine possible numbers.
• Click on one of the numbers once to mark it as a possible solution of that cell. All marked numbers will remain visible when you move the mousepointer out of the cell.
• Click on a marked number once again to remove that mark.
• Click on a number twice (double-click) to mark that number as the solution of that cell. The number will increase in size.
• Click on a solved number twice again to remove the solution and return to the state prior to marking that cell as solved.
• Cells containing the initial numbers cannot be changed.

What do the buttons on the right do?

• Click on "Start a new sudoku" to start over with a new sudoku.
• Click on "Undo" to undo your last action. Click "Redo" to undo your last undo action.
• Click on "Show mistakes" to mark all cells that contain an incorrect solution.
• Click on "Give me a hint" if you want the computer to show you the solutions of a random cell. An explanation will also be given. If the sudoku contains a mistake, no hint will be given.