Sudoku (pronounced soo-dough-coo with the emphasis on the first syllable) is a simple number-based puzzle/game played on a 9×9 grid that is divided into 3×3 subgrids. The goal is to enter a number from 1 to 9 in each cell so that each number appears only once in each column and row. In addition, each 3×3 subgrid may only contain one of each number. At the start of a new game, Sudoku fills in a random selection of cells for you - the number of cells depends on the difficulty level you use. Click in any of the empty cells or use the arrow keys to highlight individual cells and press a number from 1 to 9 to fill in the cell. To clear a cell, press 0, Delete, or Backspace. When you have successfully completed all subgrids, the entire puzzle is highlighted until you start a new game. As you work to complete the puzzle, you can display possible solutions inside each cell by holding the Shift key and pressing each number in turn. Repeat the process to remove individual numbers, or press a number without the Shift key to replace them with the actual number to use.