Common Denominator

Common Denominator is the 6th episode of Season 4 and the 46th episode overall.

When Lee and Biffy's investigation of the teachers leads them to Brandy's apartment, they start to discover why so many teachers live there.

Plot
The episode opens with Lee and Biffy falling down a hole. The reason that they're falling is because their investigation of the school's teachers has led Lee to believe that they're clones, and evidence found in some old paper files has led them to Brandy's apartment building, where all of the teachers (except Sue Ping) live. Admittedly, this isn't the only thing going on in Lee's life; Cam's suspension, Holger's possible superpowers, and The Serpent's general creepiness are all weighing heavily on his mind. However, at present he is concerned with where the tube leads. When he exits, however, he finds himself in a strange room filled with pods that contain sleeping teachers.

It all started earlier that day, when Lee decided to probe his mother about the odd coincidence of every teacher in school having the same birth date–and an odd one at that, February 29th. While he didn't get any answers from her, when he arrived at school he found Biffy waiting for him, hyper from too much caffeine and with an idea for investigating the teachers: asking the longest-tenured students if they've noticed anything odd about the teachers at A. Nigma High.

Lee agrees that this is a good plan and heads for class, where he finds Holger still attempting to figure out what his magic powers are. Soon, the news comes on; however, neither Chaz nor Tina are hosting, as they've been replaced by Irwin and the High Wizard of the Arcane Council of Knowledge and Power. The school president has yet another announcement to worsen the lives of her fellow students: because she's allergic to carbonated beverages, she has decreed that all drinks will be replaced with prune juice. This gives Lee a convenient excuse, as he tells Mr. Langhorne that the prune juice has caused him some digestive distress and gets a bathroom pass.

Lee takes advantage of the pass to visit the 15th graders, who are building a trebuchet under Chopper's direction. When he asks them about the teachers, Chopper is all to eager to help–by shoving him into the trebuchet and preparing to launch him. Before the 15th Graders can send Lee flying, however, Biffy arrives and rescues Lee. When Chopper complains that they have to test the trebuchet, Biffy provides him with a crumpled up locker door, and the 15th Graders send it flying. Despite the success of the machine, Biffy's main concern is why Chopper is working with such old technology.

Stinky is all too happy to tell them why: Chopper likes a girl. While Chopper is displeased by this reveal, he becomes even more displeased when Stinky tells Biffy and Lee that the girl in question is Beth. Both Lee and Biffy are surprised by this piece of information, but as it turns out, Beth has been coming to shop class and having him make her weapons for her Castle Serpents game. As she is the only girl in school who isn't afraid of him, Chopper has developed a crush on her.

Biffy is quick to take advantage of this by pretending to call Beth and set up a date between her and Chopper. Although Holger somehow detects this from halfway across the school, he cannot do anything at the moment, and the 15th Graders agree to help Lee with the information they have: the shop teacher, Mr. Petty, is fat one year and thin the next. This piques the interest of Lee and Biffy, but they cannot question him, as Mr. Petty is out sick today.

Brandy, meanwhile, has been focusing on usurping control of the Glamazons from Kimmie. Although Cam has offered to help, she's rejected his offer, as the endorsement of an ex-president doesn't carry much weight, and has instead focused on convincing Druscilla and McKenzie that she's the right choice to lead the Glams, as the vote will be cast this afternoon at her apartment. Of course, Cam is unwilling to let himself be left out of his girlfriend's life, and he calls Lee for help. Lee suggests that Cam enlist his mother's help, and when Mrs. Martinez hears that Brandy is going through some trouble she's supportive enough to break Cam's grounding and drive him to Brandy's condo.

Cam isn't the only one going there, as Lee and Biffy head to the apartments in order to talk to Mr. Petty. While in the hallway, however, they run into Mr. Langhorne, who is shocked to see them there. However, Mr. Langhorne's suspicions are buried when they offer to help him carry in his groceries. While Lee and Biffy are busy with the groceries in the kitchen, Mr. Langhorne heads to the bedroom in order to take a nap. Shortly thereafter, a whoosh is heard, along with an accompanying smell of Green Apple Splat, and as Lee and Biffy black out they see Mr. Langhorne's body slide down the bed and into a hole beneath the bed.

Back at school, Holger has taken matters into his own hands and grabbed Beth when she was in the middle of a Student Council meeting. Although Beth is at first upset to be disturbed, when she hears that a certain bald blacksmith is in love with her she agrees to go along with him, and upon hearing that he built her a trebuchet becomes intensely interested. Holger soon brings it over to them, and after they test the trebuchet by firing Holger from it, it seems that love is indeed the air–something that cheers Holger up despite the fact that he landed awkwardly on a pair of goalposts.

Cam, meanwhile, has made his way to Brandy's apartment. Upon getting in, he attempts to cast a vote, but his vote is rejected on two counts: firstly, he's too late to vote, and secondly, he's not a Glamazon and therefore does not get a vote. Even if his vote had counted, however, it wouldn't have mattered, as Druscilla and McKenzie both voted for Kimmie, giving her a clear majority. In response to Brandy's treachery, she is quick to boot Brandy out of the Glamazons, leaving Brandy a clique-less loser.

A few floors below, Lee and Biffy has figured out that the bed kicks in when it's laid on, and to ensure that its occupant is asleep dispenses some knockout gas. They decide to test this, and tie shirts around their mouths as makeshift air filters before lying down on the bed. Soon, the bed tilts, and the duo slide into a tube that ends up letting them out far underground in a room filled with pods. When they arrive, Biffy lands ungracefully and smacks into part of the machine occupying the center of the room, causing it to spit sparks; however, the more pressing concern for them is that the pods are all occupied with their teachers. Desiring answers, they open Mr. Petty's pod, and Biffy hauls him out.

This turns out to be a mistake, as Mr. Petty crumbles to dust in Biffy's hands. Shocked, Biffy tries to figure out a way to fix the disintegrated clone, but an alarm goes off, leaving them with no time to do anything. With guards likely on their way into the room, Lee decides that it's time for them to go, and soon Biffy and Lee are inching their way back up the tunnel by walking while horizontal and using their bodies as friction to keep from sliding back down.

That night, both Lee and Biffy are left with aching calves. Although they both agree that the experience was weird, Biffy has realized that since Mrs. Ping's birthday is February 29th and she teaches at A. Nigma High there is a non-negligible chance that Mrs. Ping is a clone as well. Although Lee doesn't want to believe this, he knows that he has to test his parents' bed to make sure, and he soon goes into his parents room and tries to get it to tip him into a pod room, only stopping when his parents walk in on him and ask what he's doing.

Characters

 * Lee Ping
 * Biffy Goldstein
 * Brandy Silver
 * Camillio Martinez
 * Holger Holgaart
 * Kimmie McAdams
 * Beth
 * Chopper McNeal
 * Stinky
 * McKenzie Phillips
 * Druscilla Duke
 * Mr. Langhorne
 * Sue Ping
 * Emmett  (Cameo appearance)
 * Irwin Dexter  (Cameo appearance)
 * Alfred Ping  (Cameo appearance)
 * Mr. Petty  (Cameo appearance)