What is the sky(天空)? Where is it? How high is it? What lies above it? These questions are hard to answer, aren’t they? Is the sky blue? The sky has no color. We know that there is air around the world. When planes fly, they need air to lift their wings(翼) . Planes cannot fly very high because when they go high, the air gets thinner. If we go up about 1,200 kilometers from the earth, we find there is no air. Maybe we can answer some of our questions now. The sky is space(空间). In this space there is only the sun, the moon and all the stars.