By now, chances are that you've heard of the term "flash mob".  If you haven't however, it's a group of people who get together and perform a task spontaneously in what is normally a very public place.  Normally everyone in the flash mob knows what's going on.  In the case of Improv Everywhere's  latest mp3 experiment, the participants were given instructions as the night went on.

The participants all downloaded the mp3 ahead of time and were given the place to meet but that was the extent of their knowledge.  Check out what they were instructed to do here:

For more on The Mp3 Experiment and behind the scenes photos check out Improv .