Answer: Paul and Linda McCartney

In the seventh season of the long-running animated sitcom The Simpsons, Lisa Simpson became a vegetarian. The impetus for her dietary shift was a visit to a petting zoo early in the episode where she fell in love with an adorable lamb and then later, when Marge served lamb chops for dinner, she could not bring herself to eat them.

As a general rule, the characters in the Simpsons universe don’t take on permanent changes that endure across future episodes (let alone seasons), but Lisa’s conversion to vegetarianism has remained a persistent aspect of her character, with subtle and not so subtle references appearing over the ensuing seasons.

Why did this change stick when so many other changes in characters over the years have been single-episode references? The episode featured a guest appearance by Paul McCartney and his then-wife Linda McCartney (they helped Lisa stick to her decision to become a vegetarian). In exchange for their guest appearance, Paul made the writers promise that Lisa’s change would be a permanent one and that she would remain a vegetarian for the duration of the show.