Laughing, even if you don't know the punchline

A woman riding a train starts laughing at something on her cell phone. Her attempt to maintain a proper train demeanor causes a couple other women to start giggling. From there the giddiness spreads. My favorite part is the guy who boards the train around the 1 minute mark. His puzzlement is priceless, and it only causes more people on the train car to dissolve into laughter.