If the sun was dimmer 4 billion years ago, how did the Earth stay warm? Today, we learn how just a small percent of Earth's atmosphere goes a long way in controlling global climate, both today and in the ancient past. Along the way, we'll visit an imaginary frozen Earth without any sun, a hazy giant moon of Saturn, and learn exactly what the greenhouse effect is.Extra credit: Visit a greenhouse, or find a cleaning product with ammonia.