There is little rain in desert. Because deserts are so dry, they have no "quilt" to help stop the soil from going away. As a result, they may get very hot during the day with the sun shining, but don't hold the heat overnight. Many deserts can quickly get cold once the sun goes down. Some deserts can reach temperatures of over 100 degrees F during the day and then drop below freezing (32 degrees F) during the night.

The largest hot and dry desert in the world is the Sahara Desert in Northern Africa. The Sahara is a sandy desert with great sand hills. It covers over 3 million square miles of Africa. Other large deserts include the Arabian Desert in the Middle East, the Gobi Desert in Northern China and Mongolia, and Kalahari Desert in Africa.

Animals that live in the desert are also used to needing little water. Many get all the water they need from the food they eat. Other animals keep water that they can use later. The camel stores up fat in its hump while other animals keep something they need in their tails.

Only certain types of plants can live in the terrible environment of the desert. You won't see a lot of tall trees in the desert. Most plants have a way to keep water in their leaves, or trunks so they can live a long time without water.

Now deserts cover around 20% of the world's land, but they are growing. This is called desertification and is caused by different reasons including human activities. The Sahara Desert is growing larger and larger each year. What should we do with it ?

    Today there are 6 billion people on the earth. By the year 2050 there may be 12.5 billion. There are a lot of people to feed. We need more food.

One way to produce more food is by growing stronger plants. For thousands of years，farmers have made plants better. Every season, they pick the best plants for the next season. It works very slowly. Since 1983, scientists have been able to change plants more quickly by changing their genetic material (遗传物质). Foods from plants grown in this way are called genetically modified foods (转基因食品), or GM foods.

    By changing the genetic material of a plant, it is possible to make new plants. They make plants which are strong against plant diseases. They can also help with our diseases: a kind of rice is being prepared, for example, which stops people from becoming blind.

    Rich countries produce GM foods because they are easy to grow and bring in more money. Poor countries are interested in them because they help produce more food.

    GM plants are not natural. No one knows how good or bad they are. Making GM foods is only one way of feeding people in 2050. There are strong feelings against them, because they are unnatural. They may feed people, and then hurt them or their children later. But, both rich and poor countries are very interested in their use, and they are not going to go away. In 2050, we may think differently about them.

B

    Most people hope to have a memory which helps them succeed in study, work and life. Can memory be improved? Luckily, hundreds of studies in the past 50 years have already given us a definite answer.

    A healthy lifestyle keeps our brain young and memory sharp. First of all, it is necessary to have a healthy diet. Eating more food rich in B vitamins and vitamin E, such as vegetables and lean meat, helps you have a better memory. Doing exercise can keep your brain alive, too. Proper exercise provides much oxygen for the brain. Running, riding a bike, swimming and even walking two or three times a week are helpful. Moreover, getting enough sleep is important. Only when your brain gets a good rest, can it work well. The healthier your lifestyle is, the better your memory will become. But do not expect a sudden change－it takes a long time to make a difference.

    Memory stills help you remember things better. The following three are the most widely used. Chunking is a way of remembering a piece of information by cutting it into small ones. For example, to memorize a ten-digit telephone number 2127983630, you can divide the digits into three groups: first 212, then 798, and lastly 3630. This method is far more effective than remembering a string of 10 digits. Organization means organizing information into groups of the same kind. For example, trees, grass and flowers are plants; tigers, pandas, horses and cows are animals. Imagery is remembering newly learnt information by using your imagination and connecting it with something that you are familiar with.

    However, memory skills will hardly work if you don't understand the information. What's more, only a regular review can make the memory last long. That is why students are always advised to go over what they have learnt and then use it as regularly as possible.

    Human brains are like muscles that needs nutrients and exercise to become stronger. If you make a few lifestyle changes and try some memory skills, you can certainly improve your memory.

    If there is no rain, there will be no water to drink. We can only live for about three days without water. If there is no rain, we'll have no water to brush our teeth. We will have no water to take a shower. We will have no water to wash dishes or our clothes.

    Plants and animals need water to grow. If there is no rain, they will die. We will not have any rice, any vegetables or any meat. If there is no rain, we will have nothing to eat. We can only live for about one or two weeks without food.

    If there is no rain, there will be no water in the streams or rivers. Fish will have nowhere to live. They will die. We will have nowhere to swim.

    If there is no rain, the cooks in the restaurants will have no water to cook food.

    If there is no rain, firemen will have no water to put out fires.

    You don't have to be a scientist to solve big environmental problems. One Dutch designer is finding creative ways to make a change—and he's inviting bikers to join him.

    Daan Roosgaarde, 38, is a designer from Holland. He uses creative ideas to improve city life. For his Smog (雾霾) Free Project, he designed a special tower that cleans air in Chinese parks. In Holland, he designed a shining bicycle path.

    Now Daan and his team are working with engineers at Chinese bike-sharing company Ofo to create a new product for public use. It's an air-cleaning machine connected to a bicycle and is part of the Smog Free Project. In smoggy weather, the machine would breathe in the air in front of the bike and blow out a stream of clean air toward the biker's face.

    It's no secret that air pollution is a big problem for China. In winter, it's common for thick clouds of haze (霾), caused largely by factory production, burning of coal and car emissions (排放物), to cover areas of China.

    In 2016, only one-fourth of Chinese cities met the country's air quality standards, according to a survey of 338 cities nationwide from the Ministry of Environmental Protection.

    "Of course one tower and a few bikes won't solve China's air pollution problem," Daan said, "But smog-free bikes are an exciting idea that will hopefully inspire communities to create greener cities."