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.

    Today some people call Amsterdam the "City of Bicycles" because it is a city which is flat and convenient（方便）for bicycles.

    In the 1960s, a group of cycling fans1an idea . They believed that it would be better for everybody if only bicycles were allowed in the city centre. They were2that this would help to save energy, reduce pollution and provide free public transport. The group painted hundreds of bicycles3and placed them in public places around Amsterdam for people to use.4was allowed to take them and use them for short journeys, whether he was a local or a foreigner, Wherever someone finished a journey, they would5the bike there for someone else to use. The problem was that it didn't work-thieves took all the bicycles6 weeks!

    7,more than thirty years later, the "white bike" is back in town-this time with a computer chip(芯片)to8 its every move! To take a bicycle, you have to put a special card inside. The new "White bike" is not white any more but is an unusual9with bright colours. The bikes are parked at special parking places and people who want to use them have to take them to another place that has enough room.

    There is already less traffic in central Amsterdam, 10 both locals and tourists have been using the white bikes. Thanks to the good ideas of lots of people, like the cycling fans in the 1960s, many people around the world have been enjoying city centre streets without cars for many years.

    In the past， many students practiced English with their pen friends — they sent letters to people all around the world. However， we now send emails to others and chat with people who live quite far away on the Internet.

    Here's some advice on how to succeed in your online language exchanges：

    How to start a conversation when speaking English online When you speak to somebody online， start by asking “Yes or No” questions. Use questions such as “Do you like …?” or “Can you …?” or “Have you ever …?”

    Remember your goal. You are trying to learn about the other person. Think about things you enjoy and try to find some common ground between you. And share your own experiences to show that you have similar interests. If you only ask questions， you will sound like a policeman or a policewoman!

    How to keep a conversation going “Listen” carefully. In that way， you can get to know somebody well. When you know him or her well， you will have more to talk about.

    Then ask follow-up questions. After you “listen”， use what you “hear” to learn more about that person. Use “open ended” questions， such as “What do you think of …?” or “What was the most interesting experience ...?”

    Remember some basic rules for online language exchanges.

    Be patient. Do not try to find a good friend right away.

    Be careful. Don't give any personal information on the Internet， especially your address， telephone number， and even your real name.

    March 20th is World Water Day every year. According to a report from the United Nations, more than half of the world's major rivers have serious water loss and are polluted.

    The Yellow River, the world's fifth longest river, watered some of the oldest farms on earth.

    {#blank#}1{#/blank#}Its source（源头）on Qinghai-Tibet Plateau is drying up. In most of the past 35 years, it didn't reach the sea.

    The Nile in Egypt, running 6, 650 km, is the longest river in the world.{#blank#}2{#/blank#}Now it is down to 2 billion m³ or so. Australia's Murray River reaches the sea only every other year. One fifth of America's Rio Grande has no water, either.

   {#blank#}3{#/blank#}

    Mostly it's damming（筑坝）and global warming that make rivers dry.{#blank#}4{#/blank#}Now about 45, 000 of them lie across the world's rivers. They hold back around 15% of the world's fresh water from running to the sea.

{#blank#}5{#/blank#}Warmer weather takes water away. The glacier（冰川）are becoming smaller. The source of the Yellow River is drying up because of it. Global warming has had certain effects on water sources. Droughts （干旱） happening around the world make people pay more attention to this fact.

    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.