Hip-hop is celebrating its 50th birthday. It started with a back-to-school party on August 11, 1973, in The Bronx. Clive Campbell, known as DJ Kool Herc, improvised and played snippets in a loop on two turntables to keep the music and dancing going. Amna Nawaz discussed hip-hop's evolution with Nelson George, author of “Hip-Hop America,” for our arts and culture series, CANVAS.