Lisa Marie Simpson is a fictional character in the animated television series The Simpsons. She is the middle child and most intelligent of the Simpson family. Voiced by Yeardley Smith, Lisa was born as a character in The Tracey Ullman Show short "Good Night" on April 19, 1987. Cartoonist Matt Groening created and designed her while waiting to meet James L. Brooks.

In The Tracey Ullman Show shorts, Lisa was something of a "female Bart": equally mischievous but lacking unique traits. As the series progressed, Lisa began to develop into a more intelligent and more emotional character. She demonstrates her intellect in the 1990 episode "Krusty Gets Busted" (season one), by helping Bart reveal Sideshow Bob's plot to frame Krusty the Clown for armed robbery. Many episodes focusing on Lisa have an emotional nature, such as "Moaning Lisa" (season one, 1990). The idea for the episode was pitched by James L. Brooks, who wanted to do an emotional episode involving Lisa's sadness, to complement the many "jokey episodes" in the first season.  In the seventh-season episode "Lisa the Vegetarian" (1995), Lisa permanently becomes a vegetarian, distinguishing her as one of the first primetime television characters to make such a choice. The episode was written by David S. Cohen (in his first solo writing credit) who jotted down the idea one day while eating lunch. Then-executive producer David Mirkin, who had recently become a vegetarian, quickly approved the idea. Several of Lisa's experiences in the episode are based on Mirkin's own experiences. The episode guest stars musician Paul McCartney, a committed vegetarian and animal rights activist. McCartney's condition for appearing was that Lisa would remain a vegetarian for the rest of the series and would not revert the next week (as is common on situation comedies). The trait stayed and is one of the few permanent character changes made in the show. In the season 13 episode "She of Little Faith" (2001), Lisa underwent another permanent character change when she converted to Buddhism.  Lisa plays the baritone saxophone, and some episodes use that as a plot device. According to Matt Groening, the baritone saxophone was chosen because he found the thought of an eight-year-old girl playing it amusing. He added, "But she doesn't always play a baritone sax because the animators don't know what it looks like, so it changes shape and color from show to show." One of the hallmarks of the show's opening sequence is a brief solo Lisa plays on her saxophone after being thrown out of music class. The Simpsons composer Alf Clausen said that the session musicians who perform her solos do not try to play at the second grade level and instead "think of Lisa as a really good player."

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in what episode did she become a vegetarian?