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.

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What are fossils?

    Fossils are the remains of plants or animals. For such remains to be considered fossils, scientists have decided they have to be over 10,000 years old. The word fossil comes from the Latin word “fossilis”, which means, “dig up”.  Fossils can be divided into “body fossils” and “trace(痕迹) fossils”. Body fossils are the remains of a plant or animal's body. Trace fossils are the remains of the activity of an animal, such as footprints, egg shells, and nests.

How are fossils formed?

    Some animals were quickly covered after their death by sediment(沉淀物) like earth, mud or sand. Over time, the parts of the animals that didn't rot (usually the harder parts like bones and teeth) were covered in the newly formed sediment. Then, a lot of chemical changes happened to the animals bodies. Water with minerals(矿物质) went into the bone and took the place of the chemicals in the bone with rock-like minerals. In the end we get a heavy, rock-like copy of the original object - a fossil. There weren't any changes in size and shape when an object changed into a fossil, but the fossil of a bone doesn't have any bone in it! It is chemically more like a rock! According to scientists, fossils have been found on every continent(大陆) on Earth.

    The sea looks very beautiful when the sun 1 on it. It can be very terrible when there is a 2 wind.

    The sea is very 3. It covers three quarters(3/4) of the earth. The sea is also very 4 in some places. There is one place, the sea is about 11 kilometers deep there. 5 mountain in the world is about 9 kilometers high. If that mountain was 6 the sea at that place, there would be still 2 kilometers of water 7 it!

    In most parts of the sea, there are 8 fishes and plants. Some live near the top of the sea. Others live deep down. There are also a lot of small living things. Lots of fishes live by 9 them.

    The sea can be very 10. When people go down, the sea becomes colder and colder. Only some men can go down into the deep sea. But, in 1970, five women scientists lived in the deep sea for fourteen days.

    Lately, people have entered the forests where chimpanzees live. The chimpanzees have been getting sick because of people's diseases. In addition, people are hunting chimpanzees. Their number is becoming smaller and smaller.

    Chimpanzees live in the plains and forests of Africa. They eat many kinds of foods, from plant leaves and seeds to insects and even other small animals. Chimpanzees' favourite food is fruit.

    Chimpanzees are very clever and they can use tools well. They can catch bugs by using sticks, and break nuts by using rocks.

    The world's forests are much smaller than they were before. Roads were built in deep forests, so people start to enter the forests where chimpanzees live. That may lead to many problems. One problem for chimpanzees is that they can easily catch people's diseases and die.

Chimpanzees are much closer to people than monkeys.

    As it's easier for people to enter forests, they hunt more often. Young chimpanzees are caught to be pets or for scientific research. The young ones are taken away from the forests, and the adults are killed. Chimpanzees are likely to disappear in the future.

    Once people thought that dinosaurs were brown or gray in colour, but the answer is to be found out. After examining the remains of a dinosaur, scientists say the colour of the feathers is bright. American and Chinese scientists rebuilt the feathers of a flightless(不能飞的)dinosaur

that lived about 150 million years ago, and researchers studied melanosomes(黑素体)in the feathers. The researchers compared them to the melanosomes that control the colour of feathers on modern birds. They were then able to make out the colours of dinosaurs' feathers. Richard Prum, a biologist(生物学家)at Yale University, said the dinosaur would be a beautiful creature if it were still alive. The discovery is useful to scientists in another way. It helps them better understand not only how dinosaurs looked but also how they lived in their daily life. Some researchers plan to use this way to study the colours of other feathered dinosaurs.