Have you ever been inside a greenhouse? A greenhouse is a glass building where plants are grown. It's very warm, because the glass wails let the sunlight in but keep the heat from getting out. The earth is like being inside a large greenhouse. Light from the sun warms land, water, and air. In turn, the warmed-up land, water, and air give off beat, which rises up towards the sky. Then gases in the sky are like a greenhouse's glass walls. They keep heat from going into space, and then earth stays warm. For the past 10,000 years, temperatures of the earth have not changed much. But, for the past 100 years or so, scientists have noticed the earth seems to be warming up more than usual. We call this global warming.

    Jack works in an office in a small town. One day his boss says to him," Jack, I want you to go to New York to an office there to see Mr Green. Here's the address. "

    Jack goes to New York by train. He leaves the station and thinks," The office isn't far from the station. I will find it easily. " But after an hour he is still looking for it. So he stops and asks an old lady. She says," Go straight ahead along the street. Turn to the left at the end, it's the second building on the right. "

      A few days later, he goes to the same city. But again he doesn't find the office. So he asks someone the way. It is the same old lady! She is very surprised and says, "Are you still looking for that place?"