One of the many highlights of Tuesday's Oprahpalooza happened nearly right off the top -- the massive choreographed crowd dance set to the live Black Eyed Peas performance of "I Got a Feeling."
It created quite the compelling scene, with thousands of people dancing in unison to the music, starting from the front and gradually moving north down Michigan Avenue.
Oprah, who had no idea the dance was planned, expressed surprise from the stage, gushing "wasn't that cool" repeatedly after it was over.
The dance itself was done in a flash-mob style, where dancers meet at a pre-planned location and break into their routine, to the surprise of an unsuspecting crowd.
The producer, Australian Michael Gracey, was also responsible for a famous flash mob dance at a Liverpool train station that was used for a phone commercial, but Tuesday's event was much larger in scale.
And in order to pull it off, late last month local casting agencies were enlisted to look for dancers.
Twenty-five-year-old Chris Mahlmann was one of the participants in the synchronized dance, but he's far from what you'd call a pro.
"I don't have dance experience. I just go out and dance by myself," Mahlmann said. "It was a really easy dance to learn, and it looked really good with the entire group."
In the hours before the show started, the professional dancers taught some of the people in the crowd the moves, helping to create an even bigger spectacle.
"The cool thing was, there were people from all walks of life -- 65-year-old women to 16-year old high school kids," Mahlmann said.
It's hard to tell how many people actually participated, but within the estimated crowd of 21,000 people, it may have been the majority. See for yourself when the actual show airs on Sept. 10.