How do you keep cool during summer? Air conditioners or electric fans are good 1 in modern society. But 2 that, people could only use fans.

    Over 3,000 years ago, fans began to 3 They were made of feathers (羽毛), bamboo or silk. They came in different 4 , including round and square.

    Later, with the invention of paper, folding(可折叠) paper fans became popular5 the Song Dynasty.  There are usually beautiful 6on fans. Some are landscapes (风景). Others are flowers and animals. Moreover, many great people including Su Dongpo, a poet of the Song Dynasty, and Tang Yin, a scholar (学者) and painter of the Ming Dynasty, once painted their masterpieces(杰作) on fans. This made fans not only 7 tools in daily life but also great artworks.

    So, with its value of beauty, people take fans as a 8 of status(地位). That's why in old China, emperors and scholars often held fans.

    Today, fans are also great collectors' items(藏品) and gifts. On the opening ceremony of the 2008 Beijing Olympic Games, folding fans were9 to leaders and officials (官员) of other countries. While they were 10 fans to get cool, they were also experiencing Chinese culture.

    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.

    How do people find their ways? At first, people did it by looking at the sun, the moon and stars, Later, we started to use the compass(指南针).Today, there are navigation satellite systems.

    A navigation satellite system uses groups of satellites, they send information to a receiver, such as your smart phone. Then they can find where you are.

    The earliest navigation system is the Global (全球的) Positioning System(GPS) of the United States, The US has put 24 satellites into space to make sure GPS can locate a person correctly and globally. Russia's GLONASS and European Union's Galileo can also work globally.

    The BeiDou Navigation Satellite System (BDS) is China's homegrown navigation system. It was set up in 2000. With it, China became the third country to build a navigation system after the US and Russia.

    BeiDou can find you wherever you are. It helps you go faster by finding shortcuts and using correct directions. When you are in trouble and call the police, they will know exactly where you are if you have a smart phone that uses BeiDou.

    Today, there are more than 20 BeiDou satellites above our heads. And the system is able to offer open services to the Asia— Pacific area.

    China plans to send more BeiDou 3 satellites into space. BeiDou—3's position accuracy(精确度) is less than five meters, By 2020, the BeiDou system will have 35 satellites and  begin serving Globally.

    Have you ever had trouble sleeping in a new place? Lots of people do. And now researchers from Brown University in Rhode Island think they know why. They found that one-half of the brain "remains more awake" than the other half when people are trying to sleep in a new place.

    The sleep findings were reported in Current Biology by Brown University. In their report, the researchers said many people report that they have a harder time sleeping the first night at a hotel or other places outside their home. They call it "first-night influence". "In Japan they say, 'If you change your bed, you can't sleep,'" said Yuka Sasaki, one of the report's writers. "You don't sleep very well in a new place. We all know about it."

    The researchers measured brain waves (测量脑波) for 35 volunteers over two nights in a laboratory. The two nights were a week apart(间隔).They found during the first night the left half of the brain was more active than the right half. This was during the first deep-sleep period, the researchers said.

    Sasaki said a lot of questions remain.

    Researchers did not keep measuring brain waves all night long. So, they don't know if the left half keeps "watch" all night, or whether it" works in shifts (轮换) with the right half later in the night" .They also do not know why the brain activity, at least during the first period of deep sleep, is always on the left half.

    For some, this research may be calming. It is good to know that our brain is "looking out for us" in a new place. But it may not help with sleep. That brain activity, at least according to this new research, makes it harder to get the sleep people need to wake up well rested in the morning.

    The population of the world is still going up and scientists are worried that there will not enough food for us in the middle of the century. One plant from Madagascar, the Kalanchoe fedtschenkoi, may help. It takes in carbon dioxide (二氧化碳) at night and it needs 10 times less water than food plants like rice. This means it can grow in the desert(沙漠).

    It is dangerous to eat this plant, but scientists can study two things about it: how it takes in carbon dioxide at night when other plants only do this during the day, and why it is so good at living in the desert. There will be less good land for growing food in the future because deserts are increase in size. There, growing plants will be more and more important.

    There is a possible way to take this research. To grow a desert plant to eat will be difficult, but it may be possible to use the genes(基因) from this plant and put them into food plants like beans or potatoes. And the food plants with the genes can grow in the desert. We hope the plant could be an answer to our future food.