There are many people in the world now. There will be a lot more people next century. So the earth will be very (1) ________ and there will be a little room for each person. (2) Maybe there will be only standing room on the earth in 600 years. (3) It will be a very serious problem. So man is thinking of a way to solve the problem.

The moon is the satellite (卫星) of the earth. Man will probably go to live there some day. But there is no air, no plants, and no life on the moon. So scientists are doing all kinds of experiments. They have to (4) ________ (solve) many problems till man can live on the moon. (5) Our dream will come true one day, we all think.

We all know the sun is important. It makes life on our planet possible. But how does the sun do that? It’s the sun’s energy.

Energy from the sun is called solar energy (太阳能). It comes in the form of light or heat. People have used solar energy for thousands of years. The sun’s energy can be used to heat water and even food. If you own a magnifying glass, your parents have probably warned you not to leave it in the sun. Solar energy can actually make the magnifying glass burn objects underneath it causing a fire. This type of energy from the sun is called solar thermal (热的) energy.

We can also turn the sun’s light into electricity. This is done with solar panels (板). Solar panels are made up of a material called silicon (硅). The silicon is heated and formed into very thin wafers (晶片). When the sunlight hits the solar panel, the electrons in the silicon move and flow through wires (电线). Using this technology, we can run a calculator and even power cars in an easy way.

But what happens on cloudy days? The electrons in the silicon will stop moving, which means the electricity can’t flow. But this doesn’t mean that solar energy can only be used on sunny days. People can use the batteries (电池) which are attached to (附着于) solar panels to store the electricity. So the stored electricity can be used to power cars at night or on cloudy days.

Did you know that solar energy is not as expensive as it used to be? In fact, in the last few years.solar panels have become much more affordable for people to use. Since the sun gives off more energy in one second than people have used, imagine what we could do if we used the sun’s energy to power our planet.

Men have always wondered about the earth they live on and about the ① ______ above them.

When early men looked at the night sky, they asked themselves a lot of questions. They looked at the moon, and wondered what this strange bright object in the sky was. They did not understand why it changed its shape from night to night. They looked at the stars and planets. They also wondered why some of them changed their positions, while others did not. ② 他们也思考地球的形状。Most early people believed the earth was flat.

It was not until the sixteenth and seventeenth centuries that scientists found the answers to some important questions. ④ With the help of Galileo’s telescope, people could see the sky more clearly, and find the positions of the stars and planets. Finally in 1665,Isaac Newton explained to the world what gravity (重力) was and people at last understood why they did not fall off the earth.