Why are Super Bowl ads so good for launching certain kinds of new products? Why do we all drive on the same side of the road? And why, despite laughing and crying together, do we often misread what others think? According to bestselling author and Harvard psychologist Steven Pinker, it all comes down to common knowledge, or the phenomenon that happens when everyone knows that everyone else knows something. Hear Pinker and EconTalk's Russ Roberts explore the necessary conditions for that knowledge, and how it can be both vital and dangerous to societies, depending on how it's used. They discuss, among other things, game-theory puzzles, how laughter spreads, how totalitarian regimes exploit uncertainty about who knows what (even when they don't), and why we often don't say explicitly what we really mean to say.