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"Flexes Its Muscles" is the second episode of season two, and the fifteenth overall of the original series.
The class is playing a game of volleyball, all but Ralphie, who's cleaning The Magic School Bus. Phoebe feels bad for him, but Keesha and Tim both point out that he often skips out of his class chores, often getting someone else to fill in for him. He takes a break with Liz and starts fantasizing about building a robot to do all of his chores while he plays sports. He runs to the others and tells them his idea, which Keesha laughs at. She bets Ralphie that if he can't make a robot, he has to put aside his pastimes of sports and do all the chores and Ralphie accepts the bet. The Magic School Bus suddenly starts changing form (first the steam boat, then the blimp, and finally the locomotive). So Ms. Frizzle declares an emergency and implements a field trip, much to Arnold's dismay.
Ms. Frizzle drives them to her friend and mechanic, Radius Ulna Humerus, also called R.U., who owns a body shop on top of the hill. The Magic Steam Train Bus then comes across a cliff and halts to a stop. Ralphie sees that R.U. has all the spare parts he needs to build a robot. The arm of the body shop grabs the bus and brings it to the front entrance. The Magic School Bus transforms into a pirate ship and is brought inside the body shop. R.U. comes around and Ms. Frizzle introduces the class to him. R.U. takes a look at The Magic School Bus and brings it inside his garage. While Ms. Frizzle and R.U. get ready to fix the bus, Ralphie searches for pieces to build the robot. But he is unsure of where he should start. So he goes into the garage where Ms. Frizzle and R.U. lift the bus up on a lift to fix it, and then the Magic School Bus transforms into a race car. Ralphie asks Ms. Frizzle for help on how to build a robot and R.U. who has overheard Ralphie's conversation, tells him that the only way to build a robot is by knowing how his own body works. So he lets Ralphie borrow his Megarific Intrascopic Kineto Examino Mediscope, "M.I.K.E." for short, to view how his body works.
Ms. Frizzle also adds that Ralphie should learn about structure to learn how his body works, which Ralphie eventually does using M.I.K.E. Dorothy Ann presses a button on M.I.K.E., showing Ralphie's bones through the screen, leading Ralphie to learn that the bones are his structure. So he tries to find stiff parts similar to the bones inside his body and uses mechanical parts for the robot. He attempts to command his newly-build robot to make him a sandwich. But the robot remains stationary. So Ralphie asks a second time and the other class wait, staring at it eagerly to see what happens.
But the robot continues to stand and Arnold's bones are seen through M.I.K.E. as he ties his shoes. Tim discovers that Arnold's bones bend between his elbow and arm and Dorothy Ann discovers that the joints make the bones move. So Ralphie goes to Ms. Frizzle to ask her about connections. R.U. chimes in and bends his mechanical watch, prompting the class to use the door hinge as an elbow joint (which goes back and forth rather than sideways), the shower head as a shoulder joint (which moves in any direction), and a flexible pipe for the backbone joint for the body parts of Ralphie's robot to move.
When the robot is rebuilt, Ralphie commands it to bow down to him, referring to himself as "Ralphie the Magnificent" in the process. But the robot continues standing and doing absolutely nothing, eventually falling apart in the process. When Ralphie stands in front of M.I.K.E. again, Dorothy Ann presses the red and blue button, showing Ralphie's muscles through the screen. Ralphie finally learns that the muscles make his bones move and Dorothy Ann informs that muscles can only pull on things when the class comes across a machine. Carlos states that the only way Ralphie's robot would move is if the class collaborate and pull together.
The robot is finally complete with support (metal poles for bones), flexibility (hinges for joints), and mobility (elastic bands for muscles). Ralphie tells the Ralphie Bot to clean The Magic School Bus and the robot finally comes to life. It steals Ralphie's hat and replaces the pan on its head for it. It walks away and Ralphie tries to stop it, which proves unsuccessful as the robot crashes through the wall of the garage.
Ralphie Bot uses the machine to bring The Magic School Bus down and nearly wipes out the class, Ms. Frizzle, and R.U. Ralphie Bot uses the mop (with Arnold holding on to it) to clean The Magic Race Car Bus (under Dorothy Ann's inadvertent command). Ralphie tells Ralphie Bot to "cool it", which prompts Ralphie Bot to use the water hose to spray The Magic Race Car Bus. Ms. Frizzle tells Ralphie to "get a grip", explaining that the body needs muscles to move the joints. Keesha turns off the water hose and Ralphie Bot walks over to the class, who run back inside The Magic Race Car Bus in the process. Ralphie Bot uses a broom to wash The Magic Race Car Bus. To stop Ralphie Bot, Ralphie finally takes it apart, saving everyone. Keesha tells Ralphie that she takes back what she said about the robot and the other class talk about how bones, joints, and muscles are important for robots as they are for humans. During the talk, the Magic Race Car Bus changes back into the Magic School Bus and the class cheer with happiness. Just then a space shuttle flies through the roof and inside the body shop. Ms. Frizzle shows impression as the shuttle swoops down to the ground.
- Steamboat (MSB-15)
- Blimp (second time)
- Locomotive Train
- Race Car (MSB-15)
- Arnold does not say his catchphrase in this episode due to being cut off by Ralphie, telling him not to.
- This is the only human body-themed episode not to have the class going inside the body. Instead, they use an X-Ray-like machine (M.I.K.E.).
- This episode has a subplot which features Liz building "Lizbot". Tying into Producer Says, Lizbot and Ralphiebot now both reside at R.U. Humerus's Auto Body Shop.
- R.U. Humerus hosts the Producer Says segment.