For one day each January, subway trains in cities across the world are filled with throngs of bare-legged riders.

Entering its 13th year, the No Pants Subway Ride has evolved from an obscure New York prank to a global event with several thousand dedicated participants eager to lose their pants and add a little humor to the commuters' day. 

Though its popularity continues to grow, a lot of people remain a little unclear on the purpose of the hilarious -- and quite deliberate -- display of undies that happens each year. So to help you better understand the origins of the day, we've gathered five facts you need to know about the history of the No Pants Subway Ride.

1. Its Purpose Is To Make Others Laugh

Though some assume the event supports a philanthropic cause, the day is actually dedicated to a fairly simple goal. Created in 2002 by the New York-based group Improv Everywhere, the celebration is focused solely on bringing a little humor to peoples' day. "It's meant to be a positive thing to make people laugh and smile," said Improv Everywhere founder Charlie Todd in an interview with USA Today. 

2. The First Ride Didn't Even Fill A Row Of Subway Seats

The first ride in 2002 featured only 7 participants. Each pantless member of the all-male group entered the train separately at seven consecutive stops, says the Improv Everywhere site. They acted as though they weren't aware of each other and said they just "forgot their pants" if asked. At the 8th stop, someone walked through the train selling pants for a $1. A portion of the stunt was captured by a group member in the clip above. 

3. The Movement Went International In 2008

In 2008, the No Pants Subway Ride expanded from New York to other U.S. and international cities. The group invited a total of nine cities to participate in the event that year, with rides taking place in the following locations: Baltimore, Boston, Chicago, Portland, San Francisco, Salt Lake City, Toronto, Washington, D.C., and Adelaide, Australia.

4. It Has Since Expanded To More Than 60 Cities

The 12th Annual No Pants Subway Ride in 2013 had over ten thousand participants in 60 cities worldwide. The event debuted in Shanghai and Hong Kong that year, and over 4,000 people participated in New York alone.

5. The 2006 Ride Was One Of The Most Memorable

The 5th Annual No Pants Subway Ride hit a major snag when it was brought to a halt by a 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 police custody. The charges were eventually dropped since it is not illegal to wear underwear in public in New York City.