It's called the No Pants Subway Ride. Could you do it?
The annual event calls for participants to board a subway car in January not wearing any pants while behaving as though they do not know each other.
Photos from Berlin, Germany show many of the non-participants with expressions that are less than excited about their bare-legged co-travelers.
It began as a joke by the public prank group Improv Everywhere in New York City and has since spread around the world, with enthusiasts in around 60 cities and 29 countries across the globe, according to the organization's site.
This is the 13th year for the event.