New York City (CNN) — People braved the cold in their underwear on subway trains around the world Sunday.
It was the 13th annual "No Pants" subway ride.
There were only two requirements to participate: you had to be willing to take your pants off and able to keep a straight face about it.
The event was organized by the Group Improv Everywhere. Founder of the group, Charlie Todd, said, "Annual no pants subway ride. And it's happening in over 25 countries around the world and over 60 cities. And Improv Everywhere started organizing in 2002 and Improv Everywhere is organizing the New York ride. We'll have about 4,000 people today at seven different meeting points all over the city."
The "No Pants" subway ride started in the U.S. in 2002 as a prank. It has now become a celebration of silliness.