Ever-active Michelle Obama visited The Tonight Show With Jay Leno on Monday, where she revealed one of the Secret Service's greatest challenges: Navigating crowds of children. The First Lady told Leno that she recently hosted a "Let's Move" playtime session at U.S. Ambassador Louis B. Susman's house in London during the Olympics, during which she darted underneath a giant parachute held up by about three-dozen kids, putting her out of her suited protectors' sight. "'Where'd she go?'" she asked playfully."'Where'd she go?' You know I always keep them guessing. It's like, 'She went under. Go get her.'"