Antwerp, Belgium, July 18, 2009--"Flash mob" participants pillow-fight at the Antwerp train station.

The first flash mobs—brief, prearranged gatherings of large groups of strangers, usually for bizarre activities—began in 2003, according to CNN.

This early morning mob, like most, was organized via the Internet and text-messaging. Some 200-plus participants showed up, many in pajamas.