Dr. Seuss’s publisher once bet him that he couldn’t write a book using only 50 different words. He decided to prove him wrong, and ended up with Green Eggs and Ham. Source
The 50 words are: a, am, and, anywhere, are, be, boat, box, car, could, dark, do, eat, eggs, fox, goat, good, green, ham, here, house, I, if, in, let, like, may, me, mouse, not, on, or, rain, Sam, say, see, so, thank, that, the, them, there, they, train, tree, try, will, with, would, and you.