A solar astronomer explains how the sun produces the seasons. He tells Marc that the sun is closer to Earth in winter than in summer, but the angle of sunlight is far different.REHASHED FROM THE PREVIOUS FOUR EPISODES: Trini learns about protecting the human body from the cold while climbing a mountain. Lisa brings in Vicky Johnson, an engineer who designs life-support systems for astronauts. Marc uses a popcorn model to show what happens when you heat any object, whether it's solid, liquid, or gas. ADDITIONAL: Lisa takes a hot-air balloon ride and learns how a motorcycle burns fuel. Marc checks out a solar-power station.Film insert: Netsilik Eskimos.ANIMATION: The evolution of home heating.