It's funny because you explained the joke
Sometimes the entire joke is someone else foolishly explaining the joke.
Family Guy had a person of middle eastern ethnicity that is slowly growing to understand american humor - as such, he attempts to fit in with his coworkers by explaining jokes that are made (originally Peter Griffin; and then it is incorporated into a few other bit pieces). Phelous, also has a bit where he goes 'get it? It's funny because...'. Ben 'Yatzhee' Croshaw featured a pic of a goat laughing in one of his videos, all the while explaining humor that is generally unfunny. Mike Stoklasa of Red Letter Media went on the record to say that only stupid people would laugh at the humor displayed in Jack and Jill - and that is exactly the target audience of Jack and Jill. He then explains the humor, for the sake of killing a few of your brain cells. The point is, sometimes it's funny to explain a joke after a joke is told. It can be a good recovery, and if well timed (like any humor), works. Mostly utilized by comedians that criticize other people. :)
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