The goal is easy: fill the 9×9 grid with numbers one through nine, ensuring each number is used only once in each mini 3×3 grid, row and column. Puzzle progress, however, can be very challenging. With this portable electronic Sudoku game, now you can challenge yourself wherever you go. 750 puzzles are grouped by difficulty and the “hint” and “undo” functions let you experiment with different moves. There’s also a timer to test your speed.