One day, Mr. Arnold was teaching a lesson, and things were going as normally as ever. He was explaining the story of human being to his pupils. He told them that, in the beginning, men were nomads (游牧); they never stayed in the same place for very long. Instead, they would travel about, here and there, in search of food, wherever it was to be found. And when the food ran out, they would move off somewhere else.

    He taught them about the invention of farming and keeping animals. This was an important discovery, because by learning to cultivate (耕作) the land, and care for animals, mankind would always have food steadily. It also meant that people could remain living in one place, and this made it easier to set about tasks that would take a long while to finish, like building towns, cities, and all that were in them. All the children were listening attracted by this story, until Lucy jumped up:

    "And if that was so important and improved everything so much, why are we nomads all over again, Mr. Arnold?"

    Mr. Arnold didn't know what to say. Lucy was a very clever girl. He knew that she lived with her parents in a house, so she must know that her family were not nomads; so what did she mean?

    "We have all become nomads again,” continued Lucy, " The other day, outside the city, they were cutting the forest down. A while ago a fisherman told me how they fish. It's the same with everyone: when there's no more forest left, the foresters go elsewhere, and when the fish run out, the fishermen move on. That's what the nomads did, isn't it?

    The teacher nodded, thoughtfully. Really, Lucy was right. Mankind had turned into nomads. Instead of looking after the land in a way that we could be sure it would keep supplying our needs, we kept developing it until the land was bare. And then off we would go to the next place! The class spent the rest of the afternoon talking about what they could do to show how to be more civilized (文明的).

    The next day everyone attended class wearing a green T-shirt, with a message that said "I am not a nomad!"

    And, from then on, they set about showing that indeed they were not. Every time they knew they needed something, they made sure that they would get it using care and control. If they needed wood or paper, they would make sure that they got the recycled kind. They ordered their fish from fish farms, making sure that the fish they received were not too young and too small. They only used animals that were well cared for, and brought up on farms.

    And so, from their little town, those children managed to give up being nomads again, just as prehistoric men had done, so many thousands of years ago.

    Here is some information about some of the wonders of the world. I am sure you will be interested in them.

    ThePyramids of Egypt

    There are about eighty pyramids known today. The most well-known was for Pharaoh（法老）

    Khufu. It is known as the "Great Pyramid". It's also the largest one. Workers used about 2.3 million blocks of stone to build it. They spent about 20 years finishing it.

    The Hanging Gardens of Babylon

    The garden sat on a hill. It was a large and wonderful structure（建筑物）. Many parts of the garden were high up on large columns（支柱）. There were many big and green trees with lovely flowers. Although there are many different ideas about the garden, we are not sure whether this wonder ever existed（存在）!

    TheLighthouse of Alexandria

    The Lighthouse of Alexandria was designed about 2,000 years ago. It was in Egypt, too. It was one of the ancient wonders of the world, about 135 metres high. It was once the highest building in the world. Although it doesn't exist now, many people come to see its relic（遗址）every year.

    Like humans, dogs donate blood too, and not every dog can donate blood. The dogs are screened(检查) for health first. Dogs must have the correct blood type, weigh at least 55 pounds and be younger than 8 years old. They may donate three or four pints(品脱) a year, which can help animals that are suffering from illnesses like cancer or an accident like being hit by a car. One pint can save up to three dogs.

    Owners volunteer their pets for the short procedure (程序), which requires no sedative, a kind of medicine given to calm him or her down. It's the dog that finally chooses to lie still and give. If the dogs want to get up and leave, they can.

    Sandy Lucas brought her 7-year-old black German shepherd, Rains, to the bloodmobile(流动血站) when it was parked at her small village. Sandy said she wouldn't have troubled herself with highway traffic and city parking problems to take the dog to Penn Vet's animal hospital in downtown Philadelphia, which is twice as far as her home. But the bloodmobile made it easy to find out if Rains could donate. "I was very, very excited that she had the right blood that was needed to help another dog out," said Sandy. "We'll surely do it again."

    Have you ever imagined going to other countries without your paper Passport? That will soon come true. And your paper passport will become history. You may use their new kind of passport

    It's reported that Australia will be the first country to use the cloud passport. With the cloud passport, you don't have to take a paper passport with you all the time. The cloud passport will have your name, photos and other information. And if you go to other countries, you can just check the information on the computer.

    When you take a paper passport with you, you might lose it. But with the cloud passport, you don't have to worry about that.

    Do you think the cloud passport is a great idea? Perhaps you will have it in the future, too.

    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.