(male narrator) Spring, summer, fall, winter-- the four seasons of the year. Why do we have seasons? Why do the days get colder and colder as winter approaches? It turns out it's all about the tilt-- 23.5 degrees, that is. The Earth's axis, that imaginary line running through the center of the spinning Earth, is tilted. When the Northern Hemisphere is tilted toward the sun, it receives the most direct light, and that's summer. When it's tilted away midway through Earth's orbit around the Sun, it receives the least direct light. That's winter. People in the Southern Hemisphere have summer while we have--brrr! In some places, the seasons don't seem to change much. The strong intensity of sunlight striking the Earth at the equator is pretty constant, so it's usually hot. We use words like "winter" and "summer" to name our seasons, but Costa Rica has a rainy season. The Sahara has a dry season. India has a monsoon season because of predictable changes in wind patterns. Cycles in nature are regulated by the seasons. When it's warm, plants grow and animals have babies. When it's cold, some animals migrate to warmer areas, crops are harvested, leaves fall. It's as if nature takes a winter nap and wakes up to start again. What's your favorite season?