Children's literature can be traced to stories and songs, part of a wider oral tradition, that adults shared with children before publishing existed. The development of early children's literature, before printing was invented, 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 1400s, a large quantity of literature, often with a moral or religious message, has been aimed specifically at children. The late nineteenth and early twentieth centuries became known as the "Golden Age of Children's Literature" as this period included the publication of many books acknowledged today as classics.