The line between opera and musical is not always easy to draw. Unlike most operas, musicals usually feature spoken dialogue; its songs are set pieces rather than advances in the action. Musicals make use of popular styles; in fact, in the first half of the 20th century, musicals served partly as vehicles for popular songs. The musical grew as two streams converged: operetta with its old-world romance, and vaudeville with its zippy comedy and thoroughly popular roots. One of the composers who brought the two streams together was Jerome Kern. Irving Berlin was more associated with the revue, a song-and-dance extravaganza. As musicals like those of Rodgers and Hammerstein began to lose overall popularity to movies, television, and rock and roll, the serious and the experimental came to the fore. Andrew Lloyd Webber showed a knack for transforming unorthodox ideas into popular successes, and eventually paved the way for stage shows employing pop music.