Buffalo from Buffalo, New York (i.e., Buffalo buffalo) that other buffalo from Buffalo, New York, “buffalo” (i.e. “harass”) go on to “buffalo” (harass) another group of buffalo from the city of Buffalo.
It’s the longest grammatically-correct sentence in English consisting of the same word. Because “buffalo” is a noun, an adjective, a place name, and a verb.
If you think that’s crazy…
There’s an epic mindblow of a tongue twister in Chinese called the “Lion-Eating Poet,” which consists of the single word shi.
Shi is actually the basis of a lot of different words depending on how you pronounce it. Since Chinese is a tonal language, the way that you vocally inflect shi (up, down, “flat,” or like a “roller coaster”) completely changes the meaning. (If English-learners get frustrated over the fact that to, too, and two have the same sound but different meanings, well… good luck with Chinese, a vastly more complex sound system.)