Fill in a number in every square so that every number appears exactly once in each row, each column and each block marked by thick lines.
To place a number, click in a square to select it, then type the number on the keyboard. To erase a number, click to select a square and then press Backspace.
Right-click in a square and then type a number to add or remove the number as a pencil mark, indicating numbers that you think might go in that square.
When you master the basic game, try Jigsaw mode (irregularly shaped blocks), X mode (the two main diagonals of the grid must also contain every number once), Killer mode (instead of single-cell clues you are given regions of the grid each of which must add up to a given total, again without reusing any digits), or all of those at once!