This flash mob showed up at a local mall and surprised the local shoppers with an incredible performance of Handel Messiah's Hallelujah Chorus. If you watch how they set things up, the leaders for each of the singing sections of the choir, soprano, alto, etc, are standing on tables, so that the singers can follow them. It is an amazing performance!
By the way, if you have never heard of a Flash Mob, don't worry, it is a phenomena that has arisen from the social networking abilities of the cell phone. People create a list on their phone of people interested in a particular event, such as this choral production, an organizer will set a time and place and text the event info to her list; her list then does the same to theirs, etc down the line. The people who are interested in joining in, show up... a Flash Mob.