Sudoku uses a 9 by 9 grid in which every row, column, and 3 by 3 box must contain the digits 1 to 9 exactly once. A puzzle supplies some digits and leaves the rest for the player to deduce.
How to generate a good Sudoku
A generator normally creates a valid completed grid first, then removes digits while repeatedly checking that the puzzle still has exactly one solution. Removing clues carelessly can create several valid solutions or make the puzzle impossible.
Difficulty depends on the logical techniques needed, not simply the number of given digits. A useful generator grades the puzzle by solving it with a defined set of human-style strategies. Many published puzzles also remove clues symmetrically for a balanced appearance, although symmetry is not required by the rules.
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For printable number activities, you can also use the worksheet generator.