If someone asks you, "What color is the sky?" I think you will answer "blue". I'm afraid that you are wrong. The sky has no color. It looks blue because we are looking at the blue sunlight （阳光）. It is shining（闪耀）on little bits of dust（灰尘）in the air.

Is the sky full of air? I'm sure that you have asked（已经问过）his 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（稀薄）. 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's all round the world. The sky is space（宇宙）. In space there is nothing, but only the sun, the moon, the earth and all the stars.