Edward William Dolch, Ph.D., compiled lists of the most frequently used words in children's books, organized by reading level. One of his most-used compilations lists simple words that account for the bulk of the words in children's books; these Dolch words are 220 service words (adjectives, adverbs, conjunctions, prepositions, pronouns, and verbs) together with 95 simple nouns. These words are generally taught in the first grade. All young readers should learn these words by sight. Dolch and his wife Marguerite wrote many children's books.
As a challenge, Dr. Seuss (Theodore Seuss Geisel) used only the Dolch words to write one of the most popular children's books, The Cat in the Hat.