Theodor Geisel, better known as Dr. Seuss, wrote over 44 children's books in his career. Some of his most famous works included The Cat in the Hat, Green Eggs and Ham, and The Lorax, which all contained relatively small vocabularies but conveyed important messages. Before writing for children, Dr. Seuss drew political cartoons and advertisements. He found inspiration for his unique animal characters from visiting his father's zoo during his childhood. The last book published in his lifetime was Oh, the Places You'll Go in 1990, shortly before his death at age 87.