Here is a Sudoku puzzle with an additional twist.
Each column must contain each of the numbers 1 through 9 exactly once.
Each row must contain each of the numbers 1 through 9 exactly once.
Each thickly outlined 3-by-3 square must contain each of the numbers 1 through 9 exactly once.
And the twist:
Each shaded 3-by-3 square must contain each of the numbers 1 through 9 exactly once.
Have fun!