Not all YouTube videos are made equal. Some languish in obscurity, while a tiny percentage rack up millions of hits. If you've ever wondered why that is, this explanation by Kevin Allocca, YouTube's trends manager, is well worth watching.

It's a slick and amusing presentation that he delivered at a recent TEDYouth event, but he does try and get to the bottom of what spurs virality. As he sees it, viral videos have three factors to thank for their success: taste-makers, communities of participation, and unexpectedness. But enough of me: over to him to explain. [TED]