Deserts are found where there is little rainfall or where rain for a whole year falls in only a few weeks' time. Ten inches of rain may be enough for many plants to survive (存活) if the rain is spread throughout the year, If it falls within one or two months and the rest of the year is dry, those plants may die and a desert may form.

    Sand begins as tiny pieces of rock that get smaller and smaller as wind and weather wear them down. Sand dunes (沙丘) are formed as winds move the sand across the desert. Bit by bit, the dunes grow over the years, always moving with the winds and changing the shape. Most of them are only a few feet tall, but they can grow to be several hundred feet high.

    There is, however, much more to a desert than sand. In the deserts of the southwestern United States, cliffs (悬崖) and deep valleys were formed from thick mud that once lay beneath (在……之下) a sea more than millions of years ago. Over the centuries, the water dried up. Wind, sand, rain, heat and cold all wore away at the remaining rocks. The faces of the desert mountains are always changing--very, very slowly--as these forces of nature continue to work on the rock.

    Most deserts have a surprising variety of life. There are plants, animals and insects that have adapted (适应) to life in the desert. During the heat of the day a visitor may see very few signs of living things, but as the air begins to cool in the evening, the desert comes to life. As the sun begins to rise again in the sky, the desert once again becomes quiet and lonely.

    There are many beaches in Hong Kong. Some of the beaches are the best in the world. There are some toilets (厕所) , changing rooms (更衣室) and stores to buy food or drinks on most of the beaches. People like going to the beaches and have fun swimming. You can swim there without (没有) danger if you follow these rules：

    ◆Never swim alone (独自).

    ◆Don't stay in the water for a long time.

    ◆Never swim after a meal or when you feel hungry or tired.

Remember：A red flag (旗帜) means it is dangerous for anybody to go into the water. A blue flag means that it is dangerous for children.

    Benjamin Banneker was born a few months before another great American—George Washington. Benjamin was black, but he was not a slave (奴隶). He and his mother and his grandmother were free. Benjamin's grandmother came from England. In America she got a job and worked for many years to pay for her boat trip across the ocean. After working many years, she saved enough money to buy a farm. Benjamin lived with her for a while. She taught him to read, write and do arithmetic (算术).

    Benjamin's neighbours knew that he was clever. They were not surprised when he built a large wooden clock. He made each piece after studying a small pocket watch. The clock made him famous, for it was one of the first clocks built in America. People from other places began to send hard problems of all kinds to Benjamin to settle.

    Thomas Jefferson heard of Benjamin Banneker's ability to settle hard problems. He asked Banneker to help build the city that was to be the capital of the United States—Washington, D.C. Banneker worked hard on the plans for the city. He marked where the streets and buildings—the Capitol (美国国会大厦), the White House, and many other places should be built.

    Later, L' Enfant, the Frenchman who had designed the new city, had a quarrel, and went back to France in anger. He took all of the plans away. The workmen couldn't build without any plans to follow.

    For a while it seemed that the plans for the capital might have to be changed, but Benjamin Banneker remembered the plans he had helped draw. He drew each again as he had built each piece of his clock.

    If it weren't for Benjamin Banneker, Washington,

    D.C might look very different from the way it does today.

    Do you know what makes lightning (闪电)? {#blank#}1{#/blank#}

    Lightning strikes happen when ice and water in clouds rub (摩擦) together, and cause atmospheric (大气层的) changes. When this happens, it creates a static charge (静电). Lightning can strike inside the cloud, between two clouds or between a cloud and the earth. {#blank#}2{#/blank#}

    Lightning can be very dangerous. Every year, lightning kills people because it can start big fires or because it reaches a temperature of 28,000℃. About 1,000 people are hit by lightning every year in the United States, and about 100 of them die as a result.

    {#blank#}3{#/blank#}Scientists think that there are more than three million lightning strikes every day in the world. An American man, Roy Sullivan, who worked in a park, survived from the most lightning strikes. {#blank#}4{#/blank#}And he keeps the highest world record.

    Lightning is sure not something to play with. When lightning strikes, it's best to stay inside a large building. {#blank#}5{#/blank#}Also, do not lie down on the ground, or the electricity may go through you and cause a heart problem.

    What's a spacewalk? Any time an astronaut gets out of a spacecraft(航天器) while in space, it is called a spacewalk. Astronauts go on spacewalks for many reasons. For example, experiments can be placed on the outside of a spacecraft. This lets scientists learn how being in space affects(影响) different things. By going on spacewalks, astronauts can also fix things instead of bringing them back to the earth to fix.

    When astronauts go on spacewalks, they wear spacesuits to keep themselves safe. Inside spacesuits, astronauts have the oxygen(氧气) they need to breathe and the water they need to drink. To keep the astronauts and the spacecraft safe, the astronauts must leave and go back to the spacecraft through a special door. When on a spacewalk, astronauts use safety tethers to stay close to their spacecraft. Tethers are like ropes, connect the spacewalkers with the spacecraft. They keep astronauts from floating(漂 浮) away into space. Another way astronauts stay safe during spacewalks is by wearing a SAFER. SAFER is worn like a backpack. It helps an astronaut move around in space.

    How do astronauts train for spacewalks? One way is by going for a swim. Floating in space is a lot like floating in water. Astronauts practice spacewalks underwater in a huge special swimming pool. For every one hour they will spend on a spacewalk, astronauts need to train seven hours in the pool. Another way astronauts practice for a spacewalk is by using virtual reality(虚拟现实). It looks and feels just like a spacewalk.

    Today, only three countries have finished spacewalks independently. They are Russia, the United States and China. The first person to go on a spacewalk in the world was Alexei Leonov from Russia. Zhai Zhigang is the first Chinese astronaut to go on a spacewalk. The world record of spacewalks is held by Russian astronaut Anatoly Solovyev. He has been on 16 spacewalks and spent more than 82 hours outside in space.