Mexico City, Mexico - Hundreds of people in Mexico City shed their pants and skirts last Sunday to take part in the "No Pants Subway Ride." The annual prank, held just for laughs, has become a global celebration of bare thighs designed to add a little humor to other "Tube" riders' day. There are only two requirements: a willingness to strip down and to keep a straight face while doing it.

For one day each January, "Chilangos" from all age groups join in on the stunt in the Mexican capital. Commuters, some without pants, some who joined the fun by taking their pants off en route, attracted glances, laughter, and some confused stares as they stood among other passengers in one of the world's busiest subway systems.

The shenanigans evolved from an obscure prank in 2002, organized by New York City-based prank collective "Improv Everywhere," and has since spread to more than 60 cities around the world. In places as far away as Bangkok, Jerusalem, London, Sofia, Stockholm, New York, and Washington, several thousand dedicated participants - fully clothed on top - rode the subway while showing off their panties, briefs, and boxers. Many wore sunglasses, while some donned brightly colored underwear and socks.

The high jinks went global in 2008, but the 2006 ride was definitely one of the most memorable. The caper hit a major snag when it was brought to a halt by a New York police officer. The train on which the participants were riding was taken out of service and all passengers were asked to exit the train. Eight people were handcuffed and taken into custody, but the charges were eventually dropped as it is not illegal to wear underwear in public in New York City.