The No Pants Subway Ride brought out the butts for the 16th year running.

Have you ever experienced that ubiquitous nightmare where you are in front of a crowd of people and you suddenly realize that you are wearing just your underwear? For some Philadelphia subway riders yesterday, that became a -willing- reality. Sunday marked the annual No Pants Subway Ride, a global phenomenon taking place in over 60 cities around the globe. As the name suggests, participants take to public transportation wearing no pants… or skirts, or shorts, or leggings, or any other kind of conventional bottom covering. Basically, you ride the train in your undies.

The No Pants Subway Ride brought out the butts for the 16th year running.

Philadelphia participants braved sub-freezing temperatures to take part in the event’s sixteenth year. The No Pants Subway Ride was the brainchild of “Improv Everywhere, a comedy collective that stages unexpected performances in public places,” according to NBC10. The goal of the project? To entertain people and bring them together. Improv Everywhere is based out of New York City, but celebrations of the No Pants event were being held in Paris, Australia, Canada, and London (they call it “No Trousers on the Tube”), among many other places. To put aside any doubts you might have about hygiene, know that No Pants does not equal “naked.”

In Philadelphia, this year’s Subway Ride was organized by Got Laundry, a laundry service. Participants were asked to bring a piece of gently-used clothing to donate to charity. “Wear two pairs of underwear if it makes you feel more comfortable,” the website said, giving tips to newbies. “Wearing a thong is totally [a] personal choice but please note that might offend people. Our aim is to make people laugh, not piss them off.”

In Philly, riders gathered at the Clothespin statue at 15th and Market Streets at 2:30 p.m. to get together for their ride. Notably, participants are supposed to act like strangers even if they know each other.