I've long felt this quote from Groucho Marx is one of the single most clever things ever said in the English language:
Time flies like an arrow;
fruit flies like a banana.
It's funny because your brain reaches the end and then tries to reinterpret each half in terms of the other, and hilarity ensues at the resulting mental images. But what's truly brilliant is that nearly every corresponding word and phrase in the two clauses changes its function—verbs becomes nouns, prepositional phrases becomes verb phrases, and so on. It's hard to imagine a more compact sentence with more mutability, and especially one that manages to be a good joke at the same time. You've got to hand it to Groucho; the dude was not just funny, but smart.
Are there any outstanding bits of human cleverness you'd like to share?