It’s almost that time of the year, folks. The time when people take to subways across the globe in the middle of winter sans trousers.

But why?

According to the stagers of the annual No Pants Subway Ride event, long-time viral stars Improv Everywhere, it’s simple:

Random passengers board a subway car at separate stops in the middle of winter without pants. The participants do not behave as if they know each other, and they all wear winter coats, hats, scarves, and gloves. The only unusual thing is their lack of pants.

Easy enough right? And sure to freak out/thoroughly amuse at least a few subway riders.

The event originated in New York City, but has spread to locations all over the world. Cities that have No Pants Subway Rides scheduled for this year include Amsterdam, Buenos Aires, Calgary, Madrid, Jerusalem, Sydney, Taipei, Warsaw, Lisbon, and Prague.

Each city’s details are different, but for instance, New Yorkers who wish to participate can meet at one of six different meeting points. From there, they will be broken down into groups and given a specific train car. Then, participants go about their business – read a magazine, play on their iPhone or whatever. When the car leaves the stop before the designated one, they take off their pants, put them in a bag, and get off at the next stop.

Participants stand on the platform, pantless, and wait for the next train. They then enter in the exact same place they exited. The performance involves acting as if all the other pantless people on board is just a coincidence.

But really. Why do people do this? The same reason anyone does something silly just for the hell of it. Whatever that is, of course.

People are being asked to participate on Twitter as well, by using the hashtag #NPSR.

Here’s some discussion of the first No Pants Subway Ride, set to appear in a documentary on the Improv group (currently a Kickstarter project).