Common knowledge is the secret engine of social life, letting us coordinate everything from meet-ups to markets to international diplomacy. In this fascinating talk, experimental cognitive scientist Steven Pinker explores its momentous impact, threading together stories of why autocrats fear blank signs, why central bankers mumble and why saying the quiet part out loud can wreck a friendship. With wit and wisdom, he invites us to better understand the ways we get into each other’s heads — and what it means when shared norms wobble.