A random running sessions that begin when someone yells "fun run!" At which point, everybody takes off their clothes and starts running in no particular direction until yelled at by someone who isn't participating in the fun run.
"Hey Norsoph, let's start a fun run."
"Hey, are those guys in a fun run?"
"Look at me, I'm in a fun run!"
"A guy walks into a bar and...FUN RUN!!!"