Thoughts
I've mentioned before that XKCD's are so good because they often include a secondary joke. At the time I believed that all comics would be
better with a secondary joke, but I think it works especially well for XKCD. One of the themes of XKCD is recognizing opportunities for adventure and tangents, and that theme is reinforced when the final punchline is tangentially related to the setup. #283 is a good demonstration of this.