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People take part in a No Trousers Day 2013 flashmob on the London Underground, in London, England, Jan. 13.

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A rider on the New York City subway.

Thousands of subway, train and bus riders stripped to their skivvies on Sunday as part of a worldwide “pants-free day” hosted by improv groups.

Pants-free days were launched in 2002 by Improv Everywhere in New York and have since taken off (literally). The rules are straightforward: Take off your pants, maintain a straight face, wear non-offensive* underwear. (Also, pay your fare.)

In Seattle, where Emerald City Improv hosted its third “No Pants! Light Rail Ride,” in 37-degree weather.

The improv groups ask that the pants-less remain in character, walking around their cities as if not wearing pants is completely normal.

*About that underwear: Emerald City Improv specifies: No thongs, jock straps and see-through briefs, among them.

--Isolde Raftery, Staff Writer

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Participants in the annual 'No Pants Subway Ride' transfer to another train at a subway station in Mexico City, Jan. 13.

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Passengers react to participants in the fourth annual 'No Trousers Tube Ride' on the London Underground.

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A soldier takes off his pants while riding the subway on Jan. 13, in New York City.

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Passengers at a subway station in Mexico City.