Phil conducts an experiment in which he tries to spin a pencil on its tip. He fails initially but is successful after sticking it through a circle of cardboard. He explains why it works and mentions the "smart" way to describe the phenomenom: the conservation of angular momentum.

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.

Galileo explains to his student that, if two balls are dropped at the same height, they will both hit the ground at the same time, regardless of their weight.

Sir Isaac Newton ponders why the planets orbit the sun and what allows them to maintain that orbit. Very soon after he sits down under an apple tree, Humphrey bangs his head against one of the limbs, causing an apple to fall on Newton's head. It is at this moment that Isaac Newton has an epiphany and starts to develop the concept of gravity.