Mexico City, Mexico - For the sixth year running, commuters in Mexico City showed off their underwear by taking part in the annual "No Pants Subway Ride" observed this past Sunday, just for laughs. The prank takes place on different dates in different cities around the world.

Once a year in the Mexican capital, "Chilangos" from all age groups join in on the stunt, which was organized in 2002 by New York City-based prank collective "Improv Everywhere." The annual event has since spread to more than 60 cities.

In places as far apart as Bangkok, Jerusalem, London, Sofia, Stockholm, New York, and Washington, participants - fully clothed on top - rode the subway while showing off their panties, briefs, and boxers, according to the organizers. Many wore sunglasses, while some donned brightly colored underwear and socks.

Improv Everywhere asks participants to abide by two requirements: Being willing to strip down and keeping "a straight face about it."

While the first "No Pants Subway Ride" had just seven participants, recent renditions have counted nearly 4,000 in New York and thousands more around the globe, according to Improv Everywhere.

The prank collective's founder Charlie Todd called the event a "celebration of silliness," and said its goal is simply to make others laugh and smile.

Improv Everywhere announces the date of the New York No Pants Subway Ride every year in early December. If you're interested in participating in 2017 in New York, or anywhere else in the world, sign up for their mailing list.