"Works Out" is the ninth episode of season three and the thirty-first episode overall of the animated children's series The Magic School Bus. It originally aired on December 1st, 1996.
Ralphie is training Ms. Frizzle for this year's Teacherathalon and the teacher tells Ralphie that muscles aren't everything. When the two jog outside, Ralphie comes to a stop to rest and catch his breath. Suddenly, The Magic School Bus gets a flat tire and the class uses an air pump to pump it up. When Arnold says he could use just as much air as the tire, Ms. Frizzle tells him that his lungs are part of the team, which prompts Janet to come around to inform the class about Mr. Sinew, who is competing against Ms. Frizzle in the annual Teacherathalon. He approaches her and wishes her luck in the competition. She informs the class that there is more to winning than just muscles, which are only part of the team.
The Teacherathalon starts, with Harry Arm as the commentator, and the winner takes home the trophy. The first event is the rowboat one across the lake, and the first one to the end wins. It starts and during it, Ms. Frizzle capriciously gets the hiccups. So Ralphie encourages her to hold her breath, only for Mr. Sinew to win. She tells the class that her muscles gave out and Keesha informs that rowing your boat while holding your breath isn't feasible. When Ralphie asks an erudite question of how breathing affects the muscles, Ms. Frizzle takes the class to The Magic School Bus.
While the class is in the bus, Liz activates the screen that shows the inside of Ms. Frizzle's body circulation. They see that the red liquid is the blood flowing and the sacks next to the heart are the lungs. Wanda notices that the blood goes from the heart to the lungs and reverts back to the heart, as well as everywhere else. They then get a look at Ms. Frizzle's leg muscles. After Ralphie asks again what effect holding your breath has on the muscles, Ms. Frizzle tells him that he needs a "closer look" and exits The Magic School Bus, with Tim following along with her. The Magic School Bus shrinks to microscopic size, Ms. Frizzle picks it up, and Liz accelerates it up her nose.
As The Magic School Bus makes its way inside, it heads down Ms. Frizzle's windpipe and to her left lung. The class views blood cells flowing and Dorothy Ann reminds Ralphie of the time they took a field trip inside his bloodstream when he was sick ("Inside Ralphie"). He then learns that when the air hits the purple blood cells, they suck it up and turn red, which happens every time Ms. Frizzle breathes in, which prompts them to realize that the air doesn't stop there. So The Magic School Bus follows it and travels through the bloodstream to find out where it goes. When they hear an incessant pounding sound, Ms. Frizzle informs them that they are traveling to her heart. As she narrowly makes it to the bicycle event on time, they makes it back in the bloodstream and realize that the blood is flowing faster as they move farther down to her leg. They then come across red bands that are called the muscle fiber, prompting Ralphie to order The Magic School Bus to stop. He realizes that the red blood cells are giving the air to the muscles and turning back to purple. So he dresses in a wet suit and exits The Magic School Bus to get a closer look. He finally learns that the red blood cells are giving the muscle fiber air. Arnold also says that the white stuff is oxygen for the muscle fiber, and Dorothy Ann reads in her book that oxygen is the air used for a body's life support. They then learn that the lungs breathe air, the blood cells carry the oxygen, and the heart pumps the blood to the muscles, which uses the oxygen. This prompts Ralphie to finally figure that the whole team for body circulation is lungs, blood, heart, and muscles, which work together. They also learn that they do so as the "oxygen team", which does so to put oxygen into the body, pump it into the muscles, use it up, and carry away the used-up stuff. This prompts them to learn that the muscles never got the oxygen they needed when Ms. Frizzle held her breath, so therefore, she was unable to win the rowboat event. They then learn that she had an oxygen problem and Phoebe notices white ooze multiplying throughout the muscle fiber, as well as less oxygen going into the muscle. Back at the Teacherathalon, Ms. Frizzle wins the bicycle event, evening the score to one victory a piece. Unfortunately, she becomes exhausted and falls to the ground.
Ms. Frizzle feels that she just needs a few minutes to let her muscles recover. But Janet reminds her that she doesn't have time for that and Mr. Sinew encourages her to get better. Tim asks the class what is going on inside and Ralphie responds that Ms. Frizzle's muscles stopped working due to not having enough oxygen. She tells them that the white ooze is called lactic acid, which muscles make when they have to work without enough oxygen. Since the final event is imminent and she doesn't have enough time to rest, Ralphie devises a plan for them to get enough oxygen inside her muscles as quick as possible. Liz drives The Magic School Bus and offers each kid a mop each to clean out the lactic acid. They, however, need to put more oxygen inside the muscles. So they let the air out of the tires and spray it onto the muscle fiber, rejuvenating them as Ms. Frizzle finally gets her stamina back.
The class hops back onto The Magic School Bus and exits Ms. Frizzle's body just in time to watch her win the last event of the Teacherathlon. When asked by Janet how she could prevail against Mr. Sinew, they explain that it was all thanks to the team of her body: her lungs, heart, blood, and muscles, as well as they themselves. Mr. Sinew also congratulates her for her determination and she tells him that she won't be able to train for next year's Teacherathalon, stating "air today, gone tomorrow".
- Harry Arm from "In the Arctic" makes his second appearance.
- This is Janet's third appearance for the season, fifth overall.
- This is the only episode so far to portray Janet as a secondary antagonist instead of primary. In this one, she is supporting Mr. Sinew who is the episode's main antagonist.
- Dan Marino, who voiced Mr. Sinew, is the legendary former quarterback for the Miami Dolphins. He played himself in the 1994 film Ace Ventura: Pet Detective, in which he was blamed by the fictional Dolphins kicker, Ray Finkle, for causing him to miss the winning field goal in Super Bowl XVII because Marino didn't hold the football "laces out."
- This is the second time the class go into a bloodstream. The first is in "Inside Ralphie".
- This is also the second time the class goes inside a nose. The first is also in "Inside Ralphie". However, here they directly go into Ms. Frizzle's nostril, and she inhales so they can go through her windpipe and enter her lung.
- Ralphie also made a shout out to the Marvel hero, The Incredible Hulk.
- This is the only episode where a field trip is taken inside of the teacher and The Magic School Bus drives into Ms. Frizzle's body, and with Liz in charge of The Magic School Bus the entire time.
- Also, this makes Ms. Frizzle the only adult in the TV series whose human body was explored.
- Mr. Sinew is in the "Producer Says" segment.
- Interestingly, Ms. Frizzle shows more skin in this episode than any other due to the workout clothes she wears during the Teacherathalon other than the matching sweatshirt and sweat pants. Also, her anatomy with the cardiovascular system is shown on a screen of what a woman's body looks like which would be generally disturbing for younger children to watch.