The Simpsons Episode 9.17 Lisa the Simpson

When Lisa is unable to figure out a simple brain teaser, she begins to fear that she is losing her gift of intelligence. Soon, she is noticing tons of clues that suggest her intellectual decline. Listening to Lisa's complaints, Grandpa explains to her that all Simpsons--himself, Homer, and others--started out smart and gradually experienced a "dumbening" that left them thick as bricks. Lisa decides to give over to the dumbening, emulating Homer and Bart and their brainless ways. But this makes Lisa unhappy, so to cheer her up, Homer gathers all the Simpson relatives he can find and tries to show her that they're not all idiots. Sadly, Lisa realizes that a lot of them are, in fact, stupid. But when she meets some of her female relatives, she is pleased to discover that many of them are doctors and businesswomen. With renewed confidence in her own intelligence, Lisa looks at the brain teaser again and figures it out immediately.