My friend Bob has a large police dog named Jack. Every Sunday afternoon, Bob and Jack have a walk in the park. Jack likes these walks very much.

    One Sunday afternoon, I paid a visit to my friend. I stayed there for a {#blank#}1{#/blank#} time. My friend and I had much more talk with each other than ever before. Soon it was time for them to take a walk in the park. We{#blank#}2{#/blank#}that, so Jack became worried about it. He walked around the room several times and then sat down in front of me and looked at me. But I still paid{#blank#}3{#/blank#} attention (注意) to him. I went on talking with my friend. At last, Jack could not wait any longer. He went out of the room and came back a few minutes {#blank#}4{#/blank#}. He sat down in front of me again. But this time, he held my hat in his{#blank#}5{#/blank#}. Soon I understood what Jack meant and so did my friend.

    If you are asked the question "What colour is the sky?" I believe you will answer blue. But I am afraid you are{#blank#}1{#/blank#}. The sky has no color. When we see blue, we are looking {#blank#}2{#/blank#} the blue sunlight. The sunlight {#blank#}3{#/blank#} on the little bits of dust(灰尘) in the air.

    Is the sky full of {#blank#}4{#/blank#}? I am sure that you are asked this question, {#blank#}5{#/blank#}. We know that there is air all around the world. We could not live{#blank#}6{#/blank#} air. Planes could not {#blank#}7{#/blank#} if there is not any air. If we go far enough {#blank#}8{#/blank#} from the earth, then we will find there is no air. Perhaps we can answer some of our questions now.{#blank#}9{#/blank#} is the sky? The sky is space. It is all around the world. There is nothing but the sun, the moon and the stars {#blank#}10{#/blank#}space.