Put the point of your pencil at the "0" in the upper left corner of the checkerboard. From this starting point draw through the center of all 64 squares using one continuous line of 21 straight strokes. You can draw horizontally or vertically but never diagonally or obliquely. You must never touch any square twice and never exit the checkerboard.

Furthermore, you must end your tenth stroke at the square marked "10" and end your last stroke at the square marked "21."