A children's song may be a nursery rhyme set to music, a song that children invent and share among themselves or a modern creation intended for entertainment, use in the home or education. Although children's songs have been recorded and studied in some cultures more than others, they appear to be universal in human society. Categories Iona and Peter Opie, pioneers of the academic study of children's culture, divided children's songs into two classes: those taught to children by adults, which when part of a traditional culture they saw as nursery rhymes, and those that children taught to each other, which formed part of the independent culture of childhood. A further use of the term children's song is for songs written for the entertainment or education of children, usually in the modern era. In practice none of these categories is entirely discrete, since, for example, children often reuse and adapt nursery rhymes, and many songs now considered as traditional were deliberately written by adults for commercial ends. The Opies further divided nursery rhymes into a number of groups, including Amusements (including action songs) Counting rhymes Lullabies Riddles Playground or children's street rhymes they sub-divided into two major groups: those associated with games and those that were entertainments, with the second category including Improper verses Jingles Joke rhymes Nonsense verse Macabre rhymes Parodies Popular songs Slogans Tongue-twisters In addition, since the advent of popular music publication in the nineteenth century, a large number of songs have been produced for and often adopted by children. Many of these imitate the form of nursery rhymes, and a number have come to be accepted as such. They can be seen to have arisen from a number of sources, including: Film Publishing Recording Nursery or Mother Goose rhymes The term nursery rhyme is used for "traditional" songs for young children in Britain and many English speaking countries; but this usage dates only from the nineteenth century, and in North America the older Mother Goose rhyme is still often used. The oldest children's songs of which we have records are lullabies, which can be found in every human culture. The Roman nurses' lullaby, "Lalla, Lalla, Lalla, aut dormi, aut lacte", may be the oldest to survive. Many medieval English verses associated with the birth of Jesus (including "Lullay, my liking, my dere son, my sweting") take the form of a lullabies and may be adaptations of