Fraggle Rock: The Animated Series (1987)

S1E22 The Funniest Joke in the Universe

Boober stumbles upon the Funniest Joke in the Universe, which has the magical effect of making any Fraggle who hears it burst into uncontrollable laughter. As the joke spreads, chaos ensues as Fraggles can’t stop giggling, leading to a hilarious series of events. Boober soon realizes that while laughter is great, there’s a time when it's important to be serious, prompting him to figure out how to restore balance among his friends.