Take a guess at what this sentence means - "Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo." It really has a proper meaning. And it's part of a whole group of one-word sentences.
The sentence (Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo.) has been the talk of grammarians since 1972. According to William Rapaport, its creator and a professor at the University of Buffalo, it means, "So, buffalo who live in Buffalo (e.g., at the Buffalo Zoo, which does, indeed, have buffalo), and who are buffaloed (in a way unique to Buffalo) by other buffalo from Buffalo, themselves buffalo (in the way unique to Buffalo) still other buffalo from Buffalo."
The sentence relies on a few tricks. The first is that "buffalo" is a verb as well as a noun and the name of a place. To buffalo someone is to confuse or fluster a person. There's also a missing "that." Under normal circumstances, we can sometimes drop a "that" from a sentence, as long as the nouns still make the meaning clear. For example, "things I knock down don't get back up," and "things that I knock down don't get back up," are equally clear. All-buffalo sentences muddle it up a bit.
According to Rapaport, the whole thing started in a philosophy class when he was a graduate student, and the professor demonstrated that the sentence, "Dogs dogs dog dog dogs," is grammatically correct and is grammatically analogous to the sentence, "Mice [that] cats chase eat cheese." The students noticed that the sentence was clearer than it needed to be, due to the unfortunate addition of "s" to some words. They looked for words with a plural and a verb that were the same. (You'll see this sentence recreated with the words "fish" or "people".)
They then tried to add words that could be more than just a regular noun. Since Buffalo is a proper noun, as well as a noun and a verb, they came up with "Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo." This, according to Rapaport, is analogous to, "Boston mice [that] Boston cats "Boston-chase" "Boston-eat" Boston cheese."
Over the years, Rapaport has had people write to him pointing out that he can add a few more "buffalo" to the sentence. If you're interested, here is a diagram of the whole thing.
S = Sentence
NP = Noun Phrase
RC = Relative Clause
VP = Verb Phrase
PN = Proper Noun
V = Verb
N = Noun