Sudoku-Killer Manual:

In play mode, which is the default mode when entering www.sudoku-killer.com, there are six different buttons enabling user interaction with the sudoku grid:

• "new game" starts a new game in a grade of difficulty chosen by the user in a menu opening as soon as the button is pressed. The more difficult the game, the less fields are predefined, such that the player has a harder time placing numbers without breaking one of the three basic rules of Sudoku.
• "save" gives the possibility of storing the current grid with all inserted numbers as long as the website is kept open. Up to four games can be saved.
• "load" retrieves a stored game from memory and displays it in the Sudoku grid. Of course this option requires precedent use of "save".
• "hint" is an option to help the user solve a riddle. Clicking this button adds one correct digit within the grid at a random position. If the game does not have a valid solution, this information is displayed.
• "surrender" can be used as a last option. The solution to the current game is displayed, ending this round. If the Sudoku cannot be solved due to a wrongly positioned digit, this information is displayed. In this case the right solution can be displayed be first clicking "restart" and then "surrender".
• "restart" deletes all user defined numbers from the grid, so that the player can begin the current game from scratch again.

In solve mode the user can type in a Sudoku riddle into an empty grid to make the sudoku-killer solve the riddle. There are two options available in this mode:

• "solve" calculates and displays a solution for the unfinished sudoku the user types in, as long as the Sudoku has a valid solution, of course.
• "clear" deletes all fields, enabling the user to insert a new Sudoku riddle easily.

Selecting a number is fairly easy and very convenient. Players just have to click on a field in order to open a digit menu for this field. To provide better overview and prevent annoying mistakes that can destroy some of the fun while solving a Sudoku, obviously impossible digits are not allowed to place within the grid. They are blocked, indicated by a more colorful and transparent background in the digit menu. Moving the mouse out of the digit menu makes it disappear again. Single digits can be erased by clicking the corresponding field.