Kelly Clarkson – “Mr. Know It All” – Tonight Show With Jay Leno – VIDEO 09/21/2011 mj santilli American Idol, Kelly Clarkson 27 A glamtastic Kelly Clarkson graced the stage of the Tonight Show with Jay Leno tonight to sing her fab new single. “Mr. Know It All.” As usual, Ms Clarkson sounds fantastic. We loves her. Watch a sparkly Kelly perform below. http://www.youtube.com/watch?v=gTDxCGAQSZs Kelly Clarkson