Some flash mobs are shocking due to their sheer spontaneity. This one, which took place on Michigan Avenue in Chicago on Sept. 8, 2009, may mesmerize you because of how well-choreographed it is. It took place during a live Black Eyed Peas performance of the song "I Got a Feeling" for The Oprah Winfrey Show's Season 24 kickoff party, and involved a crowd of about 21,000 people dancing in unison [source: Riley]. The dancing bug appeared to spread gradually throughout the crowd. It started with just a lone dancer in front of the stage, hopping up and down enthusiastically as everyone else stood stoically around her. Soon, a half-dozen other dancers joined in to her left, right and behind her. And the wave spread.
Australian Michael Gracey produced the event and cast several professional dancers to take part [source: Riley]. The day of the event, those professionals taught the dance to several hundred Oprah fans in the crowd, and several thousand more just caught on during the performance. Videos of the dance show Winfrey watching the spectacle with delight and, at the end, shouting, "This is the coolest thing ever!"