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 ?

    The world's first hydrogen-powered(氢动力) trains have begun running in Germany last month. The new trains will run 100-kilometer trips and can travel up to 140 kilometers an hour.

    A French railroad company called Alstom built two hydrogen-powered trains. Teams in Germany and France cooperated on the project, which was supported by the German government. The new train model, called the Coradia iLint, signals the beginning of efforts in Germany and other nations to move away from pollution-producing diesel(柴油) trains.

    Compared(与…相比较) with diesel trains, the Coradia iLint is designed to run on non-electrified train lines with low levels of noise. It uses a process that combines hydrogen and oxygen to produce electrical power. If the system produces more energy than the train needs at that time, it can store the extra energy in batteries. The only emissions(排放物)are water and steam.

    A single tank of hydrogen can run a Coradia iLint train for about 1,000 kilometers. This is very similar to the distance a diesel-powered train can run on with a single tank.

    Hydrogen-powered trains cost more than diesel trains to build. But Alstom officials say the operating costs are much lower. The company plans to provide another 14 Coradia iLint trains by the year 2021. Several other European countries have also expressed interest in developing hydrogen train systems. France has already said it wants its first hydrogen train to be operated by 2022.

    Goal(射门)! Football, or soccer, is one of the world's most popular sports. The game first came from old China, though the modern football game began on the streets of old England. The rule is simple: get the ball in the other team's goal, not using your hands or arms. With simple rules and easy equipment(设备), people can play football anywhere. But the game needs teamwork.

    The men's game came into the Olympics at the 1900 Paris Games in France. The women's game came into the Olympics at the 1996 Atlanta Games in Georgia. At the Olympics, players play for two 45-minute halves, with 11 players on each team.

    At the Olympics, we make 16 men's and 12 women's teams into groups of four teams. Each team plays against every other team in their group. The best eight teams in the men's and women's competitions move onto the quarterfinals. In the end the best one will become the winner of the game.