"Pranks and Greens" is the sixth episode of Season 21.
Groundskeeper Willie tells Bart about Andy Hamilton, a former student who was an even better prankster than Bart is. Bart tracks down Andy, now age 19 and still pulling pranks, and the two become quick friends.
After Bart is apprehended for playing pranks on the teachers of Springfield Elementary, Principal Skinner reveals to Bart that he is not the greatest prankster to ever walk the halls of that school. Bart is shocked by this revelation and sets out to discover the identity of this prankster. After sifting through countless back issues of the school paper, he discovers that there is a two week gap in its publication dating back ten years, and that there is a substantial change in Principal Skinner's demeanor and appearance before and after the gap. Groundskeeper Willie tells Bart that he was the school swim instructor in his day, and that Principal Skinner was once fun-loving and laid-back until an incident dubbed "Night of the Wigglers." Here, a prankster locked Skinner in a pool full of earthworms for an entire weekend. This experience changed Skinner's personality for the worst. Willie then tells Bart that the prankster was named Andy Hamilton.
Meanwhile, Marge is harshly criticized by other Springfield mothers (mainly by Manjula) for serving unhealthy snacks at their "Midday Mommies" meeting. She burns the family's junkfood, to Homer's horror. The family purchases organic food at a local market, but Marge meets with more criticism when she uses non-stick bakeware and plastic (seven) drinking bottles. The other mothers storm away and Marge realizes that she misses eating junk food. Homer catches Marge eating from his stash of candy and the couple indulge in junk food together.
Bart meets Andy Hamilton and discovers that he is unemployed and lives with his mother. When Lisa calls Andy "Loser", Bart gets him a job as Krusty's assistant but is outraged when Andy quits after one day. Bart gets him rehired and goes to check Andy's progress. When a truck full of earthworms drives into the studio, Bart is terrified that Andy is repeating the "Night of the Wigglers" prank (in the process of trying to stop Krusty, he ends up blurting what caused Principal Skinner to become the strict disciplitarian that he currently is on the air, much to Skinner's trauma). He discovers, however, that it is part of the show and that Andy has been hired as a writer for Krusty's show and has a girlfriend. Andy says that his pranking days are over, and thanks Bart for believing that he could grow up.
Despite Andy's success, when Lisa finds out that he is a TV writer she still calls him a loser (at this point the credits start to roll and Lisa repeats "Loser" a few more times).
Behind the Laughter
The episode was viewed by 7.03 million viewers with a rating share of 3.3/8, making it the second most viewed episode on Animation Domination after Family Guy. IGN gave it a 6.9/10 saying that "the episode had a good start, but veered into a less than impressive direction and Marge's storyline, while cute in parts, was really just filler more than anything else".