What’s a flash mob?

It’s when a large group of people suddenly assemble in a public place and perform an unusual action for a brief time. And to the surprise of Oprah and numerous viewers, one such performance was executed during the recent kickoff for the 24th season of Oprah’s show.

With a group of about 21,000 people, the choreographed dance was done to the song “I Gotta Feeling” as performed by the Black Eyed Peas on Chicago’s Magnificent Mile .

It all started with just one girl doing a rather “bouncy” dance routine in front of the huge crowd. Then gradually, just like a viral infection — the dance bug spread across to the rest of the crowd.

The event was produced by Michael Gracey who was also responsible for a famous similar flash mob dance at a Liverpool train station.

So how did they do it?

It seems that long before the show started, professional dancers collaborated and taught a very good sampling of the crowd the moves for the dance. They tried to make the participants as diverse as they really can. There were people from all walks of life, from 65-year-old women to 16-year old high school kids.

For a video on how they actually made it all work, you can watch this video from Oprah’s site.