Mr. Seaver begins teaching the class a lesson on gravity out of the textbook. He is especially confused by the book's claim that a coin and a feather will fall at exactly the same speed. After Luke challenges him to prove it, he tests the claim.
The students and Ms. Frizzle travel to Mercury, the first planet in our solar system, and learn that they weigh less because there is less gravity on the planet. They enjoy bouncing around with less gravity, and a few students take part in a contest to see who can jump higher.
Aurora is swimming in the pool when the spaceship unexpectedly loses gravity. She is engulfed in the water, as it forms into a bubble and stays in place, and tries to escape. She finally escapes to safety when the gravity in the ship is turned back on.