Without Knowing Why, This Whole Subway Car Broke Into A Fit Of Laughter

"Laughter is contagious."

We've all heard it before: laughter, like yawning, is contagious.

But to what extent? To explore that, a German group put contagious laughter to the test in 2011. The Berlin Laughter Project placed a few of its members on a train and ended up getting the entire car bent over chuckling. But, how? It's simple: they laughed.

Starting with one woman, the laughter slowly spreads throughout the train car. As The Washington Post reported in 2011, the laughter isn't just for fun: because of laughter's ability to release endorphins, it's therapeutic, too. 

Check the amazing video out below: