Margy Rochlin writes: When I was 19, a friend playing in Billy Joel's band invited me to watch them practice at a recording studio in Hollywood. While there, I was told that Joni Mitchell was rehearsing in another room down the hall. Noticing the door open, I walked in and sat down by myself on an old couch facing Joni and her band.

Amazingly, no one told me to leave. They were "jazzifying" all of Joni's old songs-as we'd soon hear on her 1974 live album, "Miles of Aisles"-and it was incredibly exciting to listen to them tweak a chord progression until they got it just right. At a certain point, during a break, Joni sat down right next to me. Feeling the need to explain my presence, I told her I was working on a project-this was totally invented-to collect things that people I admired had touched. "So you want to get some of my mojo working for you?" she asked. I nodded yes. She fished around in her purse for a while, then pulled out a MasterCard receipt for $4.15 from Pickwick Books and handed it to me. To this day, I keep it in a drawer, just in case I have a mojo emergency.