If you’ve been on social media these past few weeks, you’ve probably seen videos of the ice bucket challenge. It's grown so popular, that even celebrities are doing it. Other online sensations over the years include Korean rapper Psy’s “Gangnam Style” music video, and The Harlem Shake. But why do some Internet videos go viral, when others flounder? Wharton marketing researcher Jonah Berger is the author of “Contagious: Why Things Catch On.” Original air date 04/10/14