On Sunday, thousands of people in 60 different cities around the world took part in something called the "No Pants Subway Ride", including about 4,000 people in New York.
The whole point is to freak people out. Basically, you're supposed to get on a subway car with no pants on. And if anyone asks you what's going on, you have to pretend you just forgot to put pants on that morning.
This is footage from New York City.