There are many colors in nature. But do you think that a color has weight? I think you'll say 6/no". But I'm afraid you are wrong. If you don't believe it, you may do a small experiment(实验).

  First, put two objects with the same weight into two boxes. Then cover the boxes.Third, wrap(包裹) one box with a red piece of paper, the other one with a white piece of paper. OK. Now hold the boxes with your hand one by one. It is certain that you will think the red one is a little heavier.

 Why do you think so? A scientist found that different colors have different weights in man's mind. That is to say, every color has its own weight in our mind.

 The scientist told us that colors also have smell. Can you smell the color? Of course not. Then why did the scientist say S07 That is because every color stands for (代表) a kind of light with a certain wavelength,(波长). It reaches our brain through sense organs

 According to this discovery, scientists say that people accept the colors they like, and refuse the colors they hate. So your body and mind will be healthy by using the colors you like. Or you'll be nervous or even get ill.  For example (例如) ,  you like blue and hate red.  If you stay in a room with red windows, wallpaper and furniture（家具) for two hours, you'll feel you have been there for four hours. But if the room is blue, you'll feel you have been there for only an hour.  And, if a person walks out of a blue room and into a red room, his temperature will rise.That means our body temperature will change with different colors.

 A plant is a living thing. It uses its many parts to grow from a small seed to a large plant. Each part helps the plant stay healthy. Plants also need sunlight to grow. They get light from the sun and minerals from the soil. These things help the plant stay alive and grow. 

    There are many kinds of animals that live in the forest, such as mice, rabbits, squirrels, bears.

    Animals in the forest depend on（依靠）one another for food. Many small forest animals, such as mice, eat green plants for food, and then larger animals eat them. This is called food chain. A food chain begins with the sun. The green plants in the forest use the sun's energy to make food for themselves. It's called photosynthesis(光合作用). Mice eat flowers and seeds that forest plants produce(生产).The mice get their energy from the plants, and the mice become food for a larger animal, such as the hunting hawk. So the energy from the sun passes from the plants to the mice to the hawk. A break（中断）in any part of this food chain means some animals may not have enough food to eat.

    The leaders （领导人） of 13 countries had a big meeting in St Petersburg, Russia from November 21 to 24, 2010.The meeting was about tigers. They talked about the ways to protect the animals in their countries. They agreed to spend more money protecting tigers and they wanted to double the number of tigers in the world before 2022.

    Tiger is one of the world's most endangered animals. The World Wildlife Fund （WWF） says, there are only about 100,000 wild tigers now. And the number is becoming smaller and smaller. 'If nothing happens, tigers will face disasters,' said WWF spokeswoman Marie von Zeipel.

Nine types of tigers lived 100 years ago, but now at least three of them have died out. ‘If people leave tigers alone and don't kill them, then the number of tigers will double in 10 years,' said Jean-Christoph Vie, a French tiger expert (专家).

    China is one of the 13 countries where tigers live. There are about 50 wild tigers in China now. They are the Bengal tigers in Tibet, Indochinese tigers in China's southwest, and Siberian tigers in the northeast. The South China tigers lived in southeast China many years ago but disappeared in the 1970s. Many experts think they have already died out.

    Can animals be made to work for us? Some scientists think that one day animals may be taught to do a number of simple jobs. They say that in a film or on TV .We may see elephants, or monkeys, dogs, bears, or other animals doing a lot of things. If you watch carefully, you may find that those animals are always given something to eat in return(作为回报) for doing them. The scientists say that many different animals may be taught to do a number of simple jobs if they know they will get something to eat in return.

    Of course, as we know, dogs can be used to guard a house, and elephants can be used to do some heavy jobs. And we can also teach animals to work in factories. Apes(猿猴), for example, have been used in America to help make cars and scientists believe that these large monkeys may one day get in crops(收割庄稼) and even drive trains.

    Every school day, we use schoolbags to carry all of our things to school. They are a big part of our lives. But have you ever thought about the history of the schoolbag?

    In the Han Dynasty (汉朝), students in old-style private schools used bamboo boxes to carry books to school. People consider the bamboo boxes to be the earliest schoolbags. They were usually two or three layers (层) in the box. Kids put different things like books, brushes, ink stones (砚) and paper in the boxes.

    Hundreds of years later, people developed a new kind of schoolbag called the budai, or the "hip-pocket". The budai was lighter than the bamboo box. With the founding (成立) of the People's Republic of China, military rucksacks (军用帆布包) became popular. Many people carried them when they were in school. Ask your grandparents or parents about these bags. Maybe they can tell you about their experiences of carrying them when they were in school.

    A new style of schoolbags has appeared in recent years. Many of them have colorful and fashionable designs (时尚的设计) on them, such as pictures of cartoons or pop stars. And they are more comfortable to use. Some students pull wheeled bags filled with heavy textbooks and others carry backpacks.

    Schoolbags are still changing. Can you imagine what future schoolbags will be like?