Have you ever sat on a metro  where all the passengers were twins  sitting across from each other  making the space look like a human mirror with the poles separating the two sides? Or maybe you were a part of the fun when thousands of New Yorkers came together in Central Park to listen to the same mp3 at the same time? Who knows  maybe you've run into the comedy group "Improv Everywhere" when you saw a huge crowd looking up at the sky as if there was something really interesting there to see (UFO?)