Su Doku
Permanent link to this comic: http://xkcd.com/74/
Image URL (for hotlinking/embedding): http://imgs.xkcd.com/comics/su_doku.jpg
[[A square divided into 2x2 squares, the top-right one has an 1 in it, the bottom-right one has a 0, the two left ones are empty]]
[[Label: Binary Su Doku]]
{{alt text: This one is from the Red Belt collection, of 'medium' difficulty.}}