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xkcd #3114: Building a Fire
(imgs.xkcd.com)
Title text:
That was quicker than usual! The cabin's sprinkler system often makes it really hard to keep anything lit.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3114/
I feel like the last panel especially builds on the joke by continuing to divert expectations. It goes from humorously building too accurate of a log cabin, to coming back around to accidentally/on purpose starting a fire, thus completing the initial request.
The cabin is already on fire in panel 4. It is clear what happened. Then the words explain the joke.
The point isn't it is on fire. The point is the roundabout way of getting it to be on fire.
It’s a typical instruction delivery speech structure:
Hence the text in panel 4 completes the pattern.