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.[3] 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.