Great viral videos are hard to come by, but Volkswagen (or rather ad agency DDB Stockholm) appears to have hit the bullseye. Their new campaign “ The Fun Theory ” is a series of experiments, captured on video, to find out if making the world more fun can improve people’s behavior. The top video, Piano Stairs, has achieved over 1 million views on YouTube – I can’t count how many times friends have shared it this week. Among the experiments: does turning a set of subway stairs into a real-li