Children's literature includes stories such as fairy tales and songs, and are part of a wider oral tradition that adults shared with children before publishing existed. The development of early children's literature is difficult to trace. Even after printing became widespread, many classic "children's" tales were originally created for adults, and later adapted for a younger audience. Since the fifteenth century, much literature has been aimed specifically at children, often with a moral or religious message. The late nineteenth and early twentieth centuries are known as the "Golden Age of Children's Literature" because many classic children's books were published then. This category includes picture books, short stories intended to be read to youngsters, books for early readers, through elementary and middle grades, and up to young adults.