The level of difficulty increases gradually through nine different levels. Before you know it, you have learned sudoku! Comes with 60 free puzzles for learning to play. More puzzles are available to buy if you want to keep playing once you have mastered the free puzzles. - Nine levels of difficulty - Pictures that explain the rules - Puzzle book view shows your progress - Three puzzles sizes (4x4, 6x6, and 9x9) - 4500 puzzles available (60 are free) - Great exercise for your brain!