With its 9 x 9 grid, this number puzzle is the biggest challenge since the Rubik's Cube. The empty squares must be filled in such a way that the numbers from 1 to 9 appear only once in each row, each column, and each 3 x 3 block. And the best part is: You don't have to be a math whiz or a numbers freak to love Sudoku. There are 5 levels of difficulty.