Every hit you blast in the car and on the beach starts somewhere. Sometimes it's with a demo singer in a studio, who lays down a guide track that helps one of your favorite stars spin it into gold. But sometimes it's a fellow star, who helped write the song and has their own demo laying around that has its own special magic.
Case in point: this week a demo co-written by Katy Perry and Sia of the Diplo-produced "Passenger" leaked online. The track eventually ended up on Britney Spears' 2013 Britney Jean album, after the crew decided it was better suited to Britney. Happens all the time.