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.

    We have known for a long time that flowers of different plants open and close at different time of day. Yet no one really understands why flowers open and close like this at special times. It is not as simple as we might think, as new experiments(实验) have shown. In one experiment, flowers were kept in darkness. We might expect that the flowers, without any information about the time of the day, did not open as they usually do. In fact, they continued to open at their usual time. This shows that they have some mysterious (神秘的) way of knowing the time.

    Their sense of time does not depend on information from the outside world; it is, so to speak, inside them, a kind of “inner clock”. This discovery may not seem to be very important. However, it was later found that not just plants but also animals including man have this “inner clock”which controls(控制) working of their bodies and their activities.

    Human beings, then, are also controlled by this mysterious power. Whether we wish it or not, it affects such things in our life as our need for sleep, our need for food. And our ability to concentrate(集中).

    Everybody needs water. Everything needs water. We use water to wash the bowls(碗), clothes and ourselves. We use water to plant trees, flowers and crops(庄稼). Where does all water come from? It comes from clouds.

    Some of rain goes into the ground. Many people get it under the ground. Some rain water goes into the hills. It runs into lakes and rivers. Many people get it there.

    If you look at the top of your phone, you'll usually see a little symbol that says 3G or 4G.The "G" stands for the "generation (代)" of your mobile phone network. But these symbols will become things of the past after 5G networks fully arrive.

    On March 30, Shanghai vice-mayor Wu Qing made the first 5G video call on a Huawei Mate X smartphone, CGTN reported. Shanghai has also become the first city in China to start testing 5G networks.

    About 100 times faster than 4G networks, 5G will let people download and upload data faster than ever before. But 5G won't just bring faster mobile Internet. People can use it to do many other things as well.

    For example, 5G will help to make self-driving cars safer. Today's self-driving test cars have one problem-lag (延迟). When the car "sees" an obstacle (障碍物), it sends he information to a data center and receives instructions. However, it takes time to send and receive the information with this kind of lag, the cars might crash because they don't receive instructions in time. With 5G, this lag will be greatly lowered, making the cars safer according to CNN.

    5G could also be used to power the Internet of Things (IoT). This means a large online network that connects all things and people. Fast Internet speeds will be the key to developing this technology. The IoT could be used in a lot of ways. For example, with IoT, your fridge could automatically (自动地) place an order online for eggs when it finds that there are no eggs left.

An invention in front of your eyes

    What is the most important invention of all time? Is it the written word that helps us to record information? Is it the telephone that lets us talk to people from all over the world? Is it the car that lets us travel at great speeds to meet others? Umberto Eco, an Italian writer, thinks it is something quite different. He says the most important invention might be bright in front of your eyes. He is talking about a simple pair glasses.

    Modern eye glasses were invented around 1000years ago. They were more than just a convenient way to read. Before glasses, writers, scientists or teachers who could not see properly had to memorize everything. After about ten years of poor eyesight, they could not remember well enough and could not work. Glasses meant that people could work in their old age.

    People who wanted to remember some information could write it down and read it. Eye glasses are one of the world's greatest inventions and were probably worn by the people who designed the first telephones and cars.

    There are many animals in danger, such as pandas, elephants, and South China tigers.

    The South China tiger is the smallest tiger in Asia. It is also one of the rarest tigers in the world. Scientists think that there might be a few South China tigers living in the wild, but they are still not sure about it.

    The South China tiger is special in its face, but it shares the height with the common tigers. In the past, the South China tigers lived in the forests. But today more than half of them are living in zoos. If there are not safe places for them, they will die out soon. Baby South China tigers need their mothers' milk in the first two months. The mother teaches her babies how to hunt.

    Scientists think that people should do something to protect the rare animals. If people take action together, these animals will have a better place to live in. We'll also live in a better world.