Common knowledge is created by public information, such as a broadcasted statement. The story of “The Emperor’s New Clothes’’ illustrates the logic. When the little boy shouted that the emperor was naked, he was not telling them anything they didn’t already know, anything they couldn’t see with their own eyes. But he was changing their knowledge nonetheless, because now everyone knew that everyone else knew that the emperor was naked. And that common knowledge emboldened them to challenge the emperor’s authority with their laughter.

The story reminds us why humor is no laughing matter — why satire and ridicule, even when puerile and tasteless, are terrifying to autocrats and protected by democracies. Satire can stealthily challenge assumptions that are second nature to an audience by forcing them to see that those assumptions lead to consequences that everyone recognizes are absurd.

That’s why humor so often serves as an accelerant to social progress. Eighteenth-century wiseguys like Voltaire, Swift, and Johnson ridiculed the wars, oppressions, and cruel practices of their day. In the 1960s, comedians and artists portrayed racists as thick-witted Neanderthals and Vietnam hawks and nuclear cold warriors as amoral psychopaths. The Soviet Union and its satellites had a rich underground current of satire, as in the common definition of the two Cold War ideologies: “Capitalism is the exploitation of man by man; Communism is the exact opposite.”

We use barbed speech to undermine not just political dictators but the petty oppressors of everyday life: the tyrannical boss, the sanctimonious preacher, the blowhard at the bar, the neighborhood enforcer of stifling norms.