Many of the verses used by children have an element of transgression, and a number have satirical aims. The parody of adult songs with alternative verses, such as the rewriting of "While shepherds watched their flocks by night" to "While shepherds washed their socks at night" and numerous other versions, was a prominent activity in the British playgrounds investigated by the Opies in the twentieth century.[28] With the growth of media and advertising in some countries, advertising jingles and parodies of those jingles have become a regular feature of children's songs, including the "McDonald's song" in the United States, which played against adult desire for ordered and healthy eating.[29] Humour is a major factor in children's songs.