Have you seen them? Flash mobs, those random eruptions into song and dance in public places worldwide, sometimes by thousands of participants who appear to be going about their business as usual and suddenly begin a choreographed performance before dispensing back into the crowd, are popping up everywhere. Videos of these fun events are going viral, receiving tens of millions of hits from viewers as word spreads through emails, blogs, and Facebook about this growing phenomenon.

One very popular example of a flash mob is the March 2009 video that featured over 200 participants in Antwerp, Belgium, Central train station who spontaneously broke into choreographed dance steps to the  Sound of Music song “Do Re Mi” while passers-by watch in amazement. The video has received more than 21 million hits.