Keith Richards was left bewildered when Christina Aguilera joined THE Rolling Stones to perform a song during their new Martin Scorsese-directed concert film - because he had no idea who she was. The guitarist looks a little puzzled on film as Aguilera takes the stage for a rendition of Live With Me in new rockumentary Shine A Light, and he freely admits he thought she was a fan invading the stage. Richards says, "This girl came out and I was like, 'Who is that?' I had no idea. "I'm still not sure who that is. She can sing, though. And she looked good."