Beakman explains what a rainbow is, and he goes up into the "clouds" to look at rainbows up-close. He demonstrates how a prism works and then he goes on to compare raindrops to a bunch of prisms in the clouds. Beakman makes a DIY prism with a mirror and water. He dresses like a hippie and says he is ROY G BIV to help remember the colors of the rainbow.
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.