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The annual No Pants Subway Ride took place on Sunday.
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The event is organized by the Improv Everywhere comedy collective. It started in 2002 in New York with seven participants.
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Organizers say pants-less subway rides were scheduled to take place this year in dozens of cities around the world. Philadelphia's version is sponsored by a laundry delivery service.
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Participants are told to get on trains and act as they normally would and are given an assigned point to take off their pants. They're asked to keep a straight face and respond matter-of-factly to anyone who asks them if they're cold.
Photos: Thousands turn out for 'No Pants Subway Ride' in New York