Blues had been around a long time before it became a part of the first explosion of recorded popular music in American history. This came in the 1920s, when classic female blues singers like Ma Rainey, Bessie Smith and Mamie Smith grew very popular; the first hit of this field was Mamie Smith's "Crazy Blues". These urban blues singers changed the idea of "popular music" from being simple songs that could be easily performed by anyone to works primarily associated with an individual singer. Performers like Sophie Tucker, known for "Some of These Days", became closely associated with their hits, making their individualized interpretations just as important as the song itself.