At the end of Alan Moore and Brian Bolland’s 1988 one-shot Batman: The Killing Joke, the Joker lays out his worldview in plain terms. After testing his theory that “one bad day” could make even the most decent person into a madman like himself, the Joker tells his nemesis, “It’s all a joke! Everything anybody ever valued or struggled for… it’s all a monstrous, demented gag!”

Even more than the grisly sights of The Killing Joke—in which the Clown Prince of Crime sets out to prove his theory by brutalizing Batgirl Barbara Gordon and tormenting her father Commissioner Gordon—that line has set the course of Joker stories of the past several decades. Fans and creators alike try to push the Joker to edgier extremes, forgetting how Batman answers when his nemesis asks why he isn’t laughing. “Because I’ve heard it before,” he responds. “And...