Commuters in London today take part in the annual No Trousers On The Tube Day

What began as a prank by a handful of people on New York City's mass transit system in 2002 has grown into an international celebration of silliness.
Participants are supposed to behave as if they don't know each other and act normally, as if there was nothing unusual about their attire. The  "celebration of silliness," is a global movement which encourages commuters in New York, Berlin and Prague to board different train carriages and behave just like normal train passengers, minus their pants.