Improv Everywhere, the comedy collective behind various stunts such as the No Pants Subway Ride has returned with their latest underground gag. The group threw a "surprise" Sweet 16 party at the 57th Street stop of the Q train recently, enlisting willing straphangers to blow up balloons, hold signs, and eat cupcakes.

More social experiment then party, really, the idea was "to see if we could get an assortment of random commuters to work together and throw a party," stunt creator Charlie Todd tells us. "Everyone was awesome! Even the 'Showtime' guys who boarded the train in the middle of the party joined in on the fun."

While riders were informed that the birthday girl "does not know this is happening, she thinks she's going to a completely different birthday," Todd noted that "the birthday girl was actually in on it. The real surprise was for the random subway passengers." But while it wasn't actually Jenny's birthday, the group was celebrating a milestone of sorts—the 15-year-anniversary of Improv Everywhere's "surprise" party on the 6 train.

"I'll never get tired of seeing people from all different walks of life come together to do something fun on a subway car," Todd said. "Thanks to everyone who was a part of it! Even if the setup was staged, the party was real, and it was a blast!"

Find more Sweet 16 photos here, along with the full list of previous Improve Everywhere pranks—coming to a crowded subway car near you!