That group in New York called ‘Improv Everywhere’ does a flash-mob series on YouTube called “Mp3 Experiments”, where a bunch of people listen to something on headphones at the same time, and follow the instructions together.
Their latest one was in Central Park, and they got FOUR THOUSAND people to do it.
First everyone had to sing the same note all at once . . . which freaked out some people in the park who didn’t know about the stunt.