What's the best thing about fucking 21 year olds?
There's 20 of them.
The Best Thing About 21 Year Olds
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Explanation
The humor comes from the unexpected twist. It initially sounds like the question is about a sexual experience, but the punchline subverts this expectation by using the number '21' as a quantity rather than an age, suggesting there are a group of 20 year olds. This makes it a bit dark and inappropriate.