If you see people on subway trains Sunday without pants on, don't be surprised.

The annual No Pants Subway Ride event is happening. The event was started in 2002 by Improv Everywhere, a New York-based comedy collective, then was picked up in Philadelphia and other cities.

The comedy group says the pants-shedding silliness will occur in dozens of cities around the world this year.

This year's Philadelphia event, which is not listed on the Improv Everywhere site, is being sponsored by Got Laundry?, a pickup and delivery laundry service.

"First, this is NOT a NUDE event," Got Laundry?'s website says. "We will simply be removing our pants. It is expected that your private bits will remain unseen. Participate at your own risk."

Participants should meet — fully clothed — at the Clothespin statue at 15th and Market Streets in Center City at 2:30 p.m. Sunday. 

Then, about 3 p.m., participants will board the subway, paying their own fares. "Randomly after boarding and a few stops into it," the website instructions say, "remove your pants."

Ray Wall, 43, the CEO of Got Laundry?, said Friday that participants will ride the Market-Frankford Line to West Philly, come back to Center City, take the Broad Street Line south, then north, and then return again to Center City to board the Market-Frankford again, this time heading east.

After riding for about two hours, they will end up at Lucha Cartel, 207 Chestnut St., for an after-party from 5 to 7 p.m. 

The purpose of all this? "To make people smile, surprise them, and to have a fun, playful moment as adults," said Wall, of Lansdowne.

There's also a charitable component. As it has done in the last two years, when it also sponsored the event, Got Laundry? is hosting a clothing drive. People are asked to bring an extra pair of pants to the Clothespin just before the ride begins. The pants will be donated to local shelters and clothing drives for homeless people.

In an emailed statement Friday, a SEPTA spokesman said: "SEPTA is not involved with this event, however, participants have used SEPTA in connection with the event for several years now. We have not had any problems reported previously, and we don't expect there to be any this year. 

"Participants are reminded that indecent exposure is not permitted, and they cannot infringe on or interfere with other riders."

As for the after-party, Alex Watson, a manager at Lucha Cartel, said participants don't need to be pants-less to get drink specials. "It's cold out. They can absolutely put pants on."

For more info, visit: www.npsr.gotlaundry.com.