They Saved Lisa's Brain

Stephen Hawking as himself

"They Saved Lisa's Brain" is the twenty-second episode of The Simpsons' tenth season. It first aired on the Fox network in the United States on May 9, 1999. In the episode, after writing a thoughtful letter to the Springfield Shopper, Lisa is invited to join the Springfield chapter of Mensa. When Mayor Quimby later flees Springfield, the group takes control of the town, hoping to improve the lives of Springfieldians through the rule of the smartest. Meanwhile, Homer poses for a series of erotic photos.

"They Saved Lisa's Brain" was directed by Pete Michels and written by Matt Selman, although the idea for the episode was pitched by former staff writer George Meyer. It featured the first official appearance of Lindsey Naegle and was the first episode in which theoretical physicist and cosmologist Stephen Hawking guest-starred as himself. The Simpsons staff wanted Hawking to guest-star because they needed someone who would be smarter than all of Springfield's Mensa members, and because they had heard that he was a fan of the show. The episode features references to Star Trek, painter Vincent van Gogh and architect Frank Lloyd Wright, and mentions the theory of the universe being toroidal, the same shape as a doughnut.

In its original broadcast, "They Saved Lisa's Brain" was seen by approximately 6.8 million viewers, a very low amount for the series. Following the episode's broadcast, Selman made a controversial statement about East St. Louis, which was pronounced the least livable city in the United States in the episode. While the episode received mixed reviews from critics, Hawking's appearance garnered critical acclaim, and has since appeared in several lists of best guest appearances on The Simpsons. Hawking has since appeared in three episodes, and an action figure based on his cameos in the series has been produced. Although Hawking has stated that he enjoyed guest-starring on The Simpsons, he has also mentioned that his cameos have made many people mistake him for a fictional character.