What is the sky? Where is it? How high is it? What lies above it? These questions are difficult to answer, aren't they?     Is the sky blue? The sky has no colour. We know that there is air around the world. When planes fly, they need air to lift their wings. Planes cannot fly  very high because when they go higher, the air gets thinner. If we go up about  1,200 kilometres from the earth, we find there is no air. Perhaps we can answer some of our questions now. The sky is space. In  this space there is only the sun, the moon and all the stars. In space there is   _  .
nothing air everywhere everything only the sun, the moon and all the stars