You know Improv Everywhere, the New York guys who took the "flash mob" fad and started making YouTube videos of "wacky" events: dozens of people freezing for a few minutes in Grand Central Station, mass recreations of famous dance routines, and crap like that?

Their meta-fad is spreading. This Sunday, people from Emerald City Improv are taking a pants-off light rail ride between Westlake and the airport. (This will be the third year of this ritual.) And The Stranger is offering a mystery prize to the person who can photographically document the highest number of balls peeking out of participants' underwear!

The details: Central Link Light Rail, www.emeraldcityimprov.org. Free. Sun Jan 8, leaving the Westlake Center Light Rail Station at 2 pm.