S03E18 - Separate Vocations
After taking career aptitude tests, scored by a malfunctioning computer, Lisa discovers that the occupation she is best suited for is homemaker, while Bart's test shows that he should be a policeman. Lisa is heartbroken over the result and is determined to prove the test wrong. She consults a music teacher for his opinion, but he tells her that, having inherited her father Homer's stubby fingers, she can never be a professional saxophone player. Lisa is therefore required by the test to spend the day doing chores with her mother Marge, while Bart goes on a ride-along with the police.
Lisa hates her role as a homemaker, and realizing that her future dreams have been shattered, she loses interest in being a good student. Bart enjoys spending time with the police, and he even ends up stopping Snake during a car chase. When Principal Skinner discovers Bart's new interest in law enforcement, he enlists him as a hall monitor. Bart starts handing out demerits to his classmates for minor infractions and has order restored to the school. When Lisa secretly steals all of the Teachers' Editions of the schoolbooks and reveals the teachers' lack of education, it is up to Bart to figure out who stole the books. Realizing his sister is the culprit, Bart takes the blame and returns to his life as a bad student and detention regular, while Lisa goes back to being a good student. As Bart spents his time in detention, Lisa plays her saxophone outside his classroom to comfort him.