"Gains Weight" is the eighth episode of season four and the forty-seventh episode overall of the animated children's series The Magic School Bus. It originally aired on November 1st, 1997.
A slam dunk exhibition has come to Walkerville, and one kid from Walkerville Elementary will be chose to represent the school. Everyone in Ms. Frizzle's class has been training, though Phoebe wonders what are the chances any of them will be chosen? Mr. Rhule soon comes on the intercom and announces the student who will represent the school: none other than Phoebe. She begins to worry, knowing she can't do a slam dunk. Wanda reminds her that jumping is using her muscles to pull her weight off the ground. So Phoebe gives it a try, only to drop back down. Dorothy Ann reads in her book that gravity makes certain objects fall by pulling down on them.
Ms. Frizzle then arrives and takes everyone on a trip to space (an idea inadvertently suggested by Wanda, who says that going to the moon will give less gravity), and the bus becomes a basketball area with its own controllable arena, named "Planet School Bus". The bus includes an arena and Phoebe tests her jump, only to realize that the gravity on the planet pulls down on her just like the gravity on earth. So Ms. Frizzle reduces the size of the planet and Phoebe tests her jump again, jumping higher this time. She successfully does a slam dunk, only to realize she has no idea to get down.
While Phoebe continues to hang from the rim, Carlos jokingly states that she's "got the hang of this". D.A. tells Phoebe to let go since she won't fall hard with less gravity pulling her down. The rest of the class demonstrates this fact by showing that they could lift even the heaviest objects at relative ease. So Phoebe reluctantly lets go and falls very softly down the ground. When Arnold asks why there is less gravity on the planet than there is on earth, D.A. wonders if it has something to do with the size of the planet, and Ms. Frizzle explains to the class that the lesser the planet, the lesser the gravity. The teacher offers the class a scale and Phoebe, who originally weighed 65 pounds, weighs fifteen pounds on Planet School Bus. The rest of the class tries out the scale and learn that how much of their body weight depends on how much gravity it pulls down on them. Ms. Frizzle offers them basketballs and explains that with less gravity, it takes longer for them to drop when they are thrown in the air. The class plays with the balls and Phoebe uses a ball to slam dunk on the hoop. When she asks what it would be like without any gravity, Liz shrinks Planet School Bus down to size and the class floats in the air. After she experiments with low gravity and zero gravity, Phoebe tries to go back to normal, only to accidentally switch to high gravity. The class falls inside a popcorn tank and since there is so much gravity, they can't get up. Just then, they hear a cracking sound and realize that since there is too much gravity, everything that is heavy falls through a massive hole in the court, which the rest of the class fall into as well.
Carlos jokingly states that the class have "officially brought the house down". It's up to Phoebe to try and save the class. But she can't get up since she can move due to gravity pulling down hard on her. She begins to fall into the hole with the rest of the class and falls onto the scale. Phoebe then comes up with the suggestion to beat gravity with a stronger pull up, using the rope to pull the scale up so she could reach the lever to change the size of the planet. The class pulls their weight to pull the rope down to hoist Phoebe to the lever. She manages to switch the gravity back, and they return home so she can do her slam dunk.
Ms. Frizzle introduces her at the area, and she tells everyone she won't be able to do the slam dunk they expect to see. She's booed but continues to explain that it's impossible for a kid her size to do it alone. With help from the rest of the class and a see-saw, she's able to do a slam dunk. As the audience cheer, Ms. Frizzle concludes that, "You can always count on gravity to pull you through".
- From shortest to highest jumpers: Arnold, Phoebe, Ralphie, Tim, Dorothy Ann, Keesha, Carlos, Wanda.
- Phoebe's last name is revealed in this episode when Mr. Rhule announces her name over the intercom.
- This episode shares some similarities with Plays Ball, where it has a sport, but the difference is that Plays Ball is brought to you by friction and this episode is brought to you by gravity.