" Beneath Hsieh’s unconventional approach lies a mathematical structure based on what he calls collisions. Collisions—defined as serendipitous personal encounters—are, he believes, the lifeblood of any organization, the key driver of creativity, community, and cohesion. He has set a goal of having one thousand “collisionable hours” per year for himself and a hundred thousand collisionable hours per acre for the Downtown Project. This metric is why he closed a side entrance to Zappos headquarters, funneling people through a single entrance. "

― Daniel Coyle , The Culture Code: The Secrets of Highly Successful Groups