If you've never heard of a flashmob before, it's when word gets out somehow for everyone who finds out about it to gather at a particular place and time, count down from five, and then do something weird. There have been flashmobs where everyone opened an umbrella for precisely 23 seconds, made chicken noises, turned their shirts inside out, had a pillow fight, and even briefly created a supercomputer. About five years ago, there was even a silent disco held in the stations of London's subway system and 1,000 people gathered for a flash rave in Union Square in …