If someone asks you, "What colour is the sky?" I think you will answer "blue". I'm afraid that you will be wrong. The sky has no colour. It looks blue because we are looking at the blue sunlight. The sunlight is shining on little bits of dust  in the air. Is the sky full of air? I'm sure that you have asked this question, too. We know that there is air all around the world, but we couldn't fly very high because as we go high, the air gets thinner. If we go far enough away from the earth, we find there is no air. Maybe we can answer some questions now. What is the sky? Where is it? It is all round the world. The sky is space . In space there is nothing, but only the sun, the moon, the earth and all the other stars. The sky is   _  .
a star round the earth a big ball with air full of dust space with the sun, the moon, the earth and many other stars