In the world of classic children's literature, Dr. Seuss holds an elevated position, but some parents disapprove of his habit of making up words—wumbus, yuzz and diffendoofer, for starters. These critics argue that children's books should teach English, not nonsense, and they worry that their children will have difficulty distinguishing invented words from real vocabulary. But the made-up words are much of the point. Even if it doesn't build SAT vocabulary, Dr. Seuss's nonsense verse helps teach children to focus on English sentence structures, pronunciation and rhythms. Ultimately, kids will figure out that oak trees exist and truffula trees don't. Some words, like nerd and grinch, have even leaped from Dr. Seuss's imagination into common usage and dictionaries. Not all non-standard English is substandard.