Little-known story of start of Crosby, Stills & Nash

      |Rock legends Crosby, Stills & Nash (David Crosby, Stephen Stills and Graham Nash) just kicked off a 60-city summer tour and are releasing their first DVD in more than 20 years. They talked about what it's like to be on the road again with "CBS This Morning: Saturday" co-hosts Rebecca Jarvis and Anthony Mason. And they told how "Mamma" Cass Elliott of The Mamas and the Papas brought them together.