Dreams can happen in any of the four stages of sleep. The most memorable and clearest dreams happen in the last stage of sleep (also known as REM sleep). The sleep cycle repeats about four to five times every night. So you can see how a person has several different dreams in one night. Most people only remember dreams that happen closer to the morning when they are about to wake up. That you can't remember those dreams does not mean they never happened. The stages in the sleep cycle are organized by the changes in different brain activities.

Stage 1: You are entering into light sleep. In this stage, your eyes move rapidly. We call this NREM. The body gets relaxed and your body temperature lowers. Your heart rate lowers, too. The body is preparing to enter into deep sleep.
Stage 2: Also characterized by NREM. In this stage, the body temperature and heart rate drop again. You are completely asleep.
Stage 3: Still in the NREM stage, this is a deeper sleep still.
Stage 4: In this stage of sleep, your eyes move back or forward. We call it REM sleep. This stage happens at about 90-100 minutes after the beginning of sleep. Your blood pressure rises, heart rate speeds up. The majority of your dreams happen in this stage.
These stages repeat themselves throughout a night's sleep.

    In most parts of the world, manystudents help their schools make less pollution.They join environment clubs.Inan environment club, people work together to make our environment clean.

    Here are some things students oftendo.

    No-garbage lunches.How many lunch bagsdo you throw away after lunch? Environment clubs ask students to bring theirlunches in bags that can be used again.Every week they will choose the classesthat make the least garbage and report them to the whole school.

    No-car day.On a no-car day, nobodycomes to school in a car including the students and the teachers.Cars givepollution to us, so remember:

    Walk, jump, bike, or run!

    Use your legs! It's lots of fun!

    Turn off the water! Do you know thatsome toilets can waste twenty to forty cubic meters of water an hour? In ayear, that would fill in a small river. In the environment clubs, students mendthose broken toilets.

    We love our environment.Let's worktogether to make it better.

    Air is a colorless, tasteless and invisible（看不见的）gas that is around us. An "empty" drinking glass and an "empty" room, in fact, are not really empty. Each is filled with air. When the glass is filled with water, the water pushes the air out of the glass.

    Air, as a gas, has no definite shape, but because it is matter, it takes up space and it has weight. It is easy to prove that air is something that takes up space. Stuff a piece of dry cloth（布）into the bottom of a glass so that it will not fall out when the glass is turned upside down. Push the upside down glass, holding straight up and down, into a jar of water till the glass is completely covered. When the glass is taken out of the water, the cloth is still dry. The air inside the glass shows that it takes up space and keeps the water from coming in.

Are You Right Handed or Left Handed?

    Which hand do you use when write? About 8 to 15 percent of people are left handed. They often have to use tools that are designed for right-handed people. So it is difficult for left-handers to use most tools. If you are right handed, try this experiment: Take a knife with your left hand and try cutting a potato into pieces. Don't be surprised if you feel awkward（别扭的）.

    In the past, people thought it was strange to use the left hand. Young students looked down upon their left-handed classmates. Some children were even punished for using their left hand to write. But these days parents and teachers have accepted that. In almost every school in the world, left-handed students can use their left hand to write.

    What causes people to be left handed or right handed? Experts have searched long and hard on this. They conclude（得出结论） that left-handed people are left handed for the same reason that right-handed people are right handed. One out of every ten people just is a left-handed people are right handed. One out of every ten people just is a left-hander. It's simply like the color of our eyes – some people have brown eyes, while some others have black eyes.

    However, may researchers think that left-handers and right-handers are different in some aspects. Right-handers are more talkative and outgoing than left-handers. Many right-handed people understand spoken words better. For example, after they listen to directions to a supermarket, they can find it easily. Right-handers are good at organizing people, too. They are also better basketball guards. Just ask Yao Ming.

    Research shows that left-handers are creative and artistic. Many famous performers, like Jim Carrey and Paul McCartney, are left handed. Many left-handers learn better visually（在视觉方面）. In art, both Leonardo da Vinci and Michelangelo were left handed. In music, Ludwig van Beethoven was left handed. In science, you find Newton and Einstein. The left-handed people are also really good at tennis and other single sports.

    If you prefer one hand, but you are still good at writing with the other, you are mixed. Research shows mixed-handers can remember everyday things better than other people. What did you eat for lunch two weeks ago? If you're mixed handed, you can probably remember.

    Man has a big brain（大脑）. He can think, learn and speak. Scientist once thought that men are different from animals because they could think and learn. They know now that dogs, rats, birds can learn too. They are beginning to understand that men are different from animals because they can speak.

    Animals cannot speak. They make noises when they are afraid, or angry or happy. Apes（类人猿）can understand some things more quickly than men. One or two of them have learnt a few words. But they cannot put words together to make sentences. They cannot think as we do because they have no languages. The language is a wonderful thing. Man is able to build a modern world because he has language.

    Each child can speak his own language very well when he is four or five, but no animal learns to speak. How do children learn it? Scientists don't know. They only know that man can speak because he has the brain.