Hummingbirds(蜂鸟) are one of nature's most energetic fliers and the only birds to hover(盘旋) in the air by relying on their strength alone.

    Now scientists have found that it is the ratio(比值) of the bird's wing length to its width that makes them so efficient. The discovery is helping experts compete with 42 million years of natural selection to build helicopters that are increasingly efficient.

    David Lentink, an assistant professor at Stanford University in California, tested wings from 12 different species of hummingbirds, which he sourced from museums. He placed them on a machine used to test the aerodynamics(气力学) of the helicopter blades(桨叶). Professor Lentink's team used the same machine to test the blades from an advanced micro-helicopter used by the UK's army. They found that the micro-helicopter's blades are as efficient at hovering as the average hummingbirds.

    But while the micro-helicopter's blades kept pace with the average hummingbird wings, they could not keep up with the most efficient hummingbird's wing. The wings of Anna's hummingbird were found to be about 27 percent more efficient than the man-made micro- helicopter's blades.

    While Professor Lentink wasn't surprised at nature's superiority, he said that helicopter blades have come a long way. “The technology is at the level of an average hummingbird,” he said. “A helicopter is really the most efficient hovering device that we can build. The best hummingbirds are still better, but I think it's amazing that we're getting closer. It's not easy to match their performance, but if we build better wings with better shapes, we might match hummingbirds.”

    Professor Lentink said that we don't know how hummingbirds maintain their flight in a strong wind, how they navigate(确定方向) through branches, or how they change direction so quickly. He thinks that great steps could be made by studying wing aspect ratios-the ratio of wing length to wing width. Understanding these abilities and characteristics could be a benefit for robotics and will be the focus of future experiments.

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    As we know, everyone can make mistakes, but don't worry about it. The fol-lowing is what we should do when we make mistakes.

    Just admit (承认）it. Don't think that our mistakes are caused by other {#blank#}1{#/blank#}Don't blame other people, and don't cheat ourselves, either {#blank#}2{#/blank#} In order to stop the problem from getting worse, we should act quickly to solve it. If we don't act and deal with the problem quickly, it'll only make u feel more stressed. Also it is necessary to make a plan of action and correct the mistake according to it. {#blank#}3{#/blank#}.

    {#blank#}4{#/blank#} Those who have made the most mistakes can get an A, for they are the ones who have learned the most. The more we learn from our mistakes, the cleverer we are.

    {#blank#}5{#/blank#} What caused us to make the mistake? How can we avoid making the same mistake in the future? When we act with a method to get a good result, do it that way next time. If we don't, we should stop, think about it, change it and try again.

A. Realize that making mistakes is the best way to learn.

B. Don't be afraid of failure.

C. Be brave and face the fact that we have made the mistakes.

D. Solve it at once.

E. So be away from stress.

F. So decide quickly and act.

G. Think about what we can learn from our mistakes.

    Spending money on time-saving services reduces stress and boosts(增进）happiness according to a new research, but shockingly, few of us do it.

    Whillans, a professor at HBS said, "Buying time helps to protect us from the stress in our lives caused by time pressure, and the feeling that we don't have enough minutes in the day to complete our tasks."

    The effect was clearest in the Canadian experiment, in which 60 working adults were given $40 to spend in two different ways. One weekend, they were told to spend the money on a material purchase—a gift for themselves. The next weekend, they were instructed to spend the $40 on anything that saved them time,  from paying the neighbor ' s kid to run errands (跑腿）to taking an Uber instead of a bus.

    "On the day they made the time-saving purchase, they felt happier, in a better mood, and lower feelings of time stress than on the day they bought a material purchase, " said Whillans.

    The biggest surprise to the researchers was how few people would spend money on time-saving services. When they asked 98 working adults how they would spend a "windfall"(意外之财) of $40, only two percent named a purchase that would save them time.

    "One reason," said Whillans, "is that we're very bad at remembering how much we hate doing certain tasks once the suffering has passed. That makes us less likely to take active steps to avoid that overburdened feeling in the future. "But another possible cause is good old-fashioned guilt." If you feel guilty about getting someone to clean your house for you, then you might get less happiness from outsourcing (外包）that task, " said Whillans, "or you might just be less likely to spend your money in that way."

    The Taj Mahal (泰姬陵) is a love story, a sad and beautiful one. If it didn't exist, we would easily imagine that the story of its construction was simply a fairy tale. Three hundred years ago, there lived an Indian emperor called Shah Jahan. His wife was a beautiful and bright woman whom he loved greatly. Her title was Mumtazl Mahan: its shortened form Taj Mahan means "pride of the palace". In the year 1630 this beloved wife of the emperor died. He was so brokenhearted that he thought of giving up his throne. He decided out of his love for his wife, to build her the most beautiful tomb that had ever been seen.

    He summoned the best artists and architects from India, Turkey, Persia and Arabia and finally, the design was complete. It took more than twenty thousand men working over a period of 18 years to build the Taj Mahan, one of the most beautiful buildings in the world. The building itself stands on a marble platform 29 meters square and 67 meters high. Towers rise from each of the four corners. The Taj itself soars another 61 meters into the air. It is an eight-sided building made of white marble. The emperor planned to build an identical tomb of black marble for himself on the other side of the river connected by a silver bridge. However his son put him into a prison in the palace before he could finish, and for the rest of his life, he could only gaze across river at the tomb of his beloved wife.

    Stephen Hawking is one of the most famous scientists in this century. He was born in 1942. He's world well-known on space and time. Stephen was searching some very big questions, such as "How did the universe begin? How will it end?"

    Stephen was a student at Oxford University. He studied math and science. Then, at the age of twenty, he became sick. He was so young, but the doctors said to his family, "He has only two more years to live." As a matter of fact, the doctors were wrong – he didn't die. He can't walk but he uses a wheelchair. He can't feed himself and get in or out of bed himself. But he refused to give in to the condition. He talks with the help of a computer. After Oxford, Stephen went to Cambridge University. Three years later, in 1965, he became a doctor of philosophy(哲学).

    Because of his serious health problems, it was difficult for him to draw or to write. So he started to think in pictures. With this new way of thinking, he became one of the most famous scientists in the world. In 1981, he met the Pope(教皇) in Rome. They talked about his ideas. Then in 1988, he wrote his first important book, A Brief History of Time. It sold more than 5.5 million copies in 33 different languages. He was once invited to China, he impressed us with his self-confidence, humorous and witty(风趣的) conversation.(谈话)