People were asked to download an mp3 but not to listen to it, instead you had to turn up in a park with it on your ipod (or whatever) & wait to be told when to press play.
what happened next was a park full of people (looks like a good hundred or so) all doing odd things in unison
What a strange but imaginative world we live in:
http://www.improveverywhere.com/mission ... sion_id=52