Whats that video called?

so a while ago i saw a video of these new yorkers who where listening to there ipods and they were following instuctions of some guy (or girl) talking to them through there head phones. the guy(or girl) would give them random tasks such as "freeze' just to get the publics reaction. i cant remember what that video was called but if someone knows what im talking about then please could you send me a link? i need it for a school ignment.