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    Every year, hundreds of millions of Monarch Butterflies(黑脉金斑蝶) from Canada and the United States journey as far as 2, 500 miles to the forests of Michoacan, Mexico, a place which has the world's largest insect migration(迁移). It's such a breathtaking sight, but as always, human greed is threatening to destroy it.

    The Monarch Butterflies start to arrive in Michoacan in late October to make their winter home in the trees high up in the mountains of the natural reserve. Once there, they gather together in large masses. These masses often become so heavy that they cause tree branches to bend or even break. But there's a purpose of all these massing — it allows the butterflies to survive in the low nighttime temperatures at these high altitudes.

    The Michoacan Monarch Butterfly Sanctuary is most impressive during the months of February and March, just before the winged insects begin their long journey home.

    Mexico's Butterfly Forest is a shelter protected by law, and one of the country's most popular sights, but that hasn't stopped people from slowly but steadily destroying it.

    Illegal woodcutting in the heart of the Monarch Butterfly Biosphere Reserve has been a longstanding problem, but criminals are rarely brought to justice. They are often set free after paying some money.

    Just last month, an even greater threat rose in Michoacan's butterfly home.  The country's largest mining corporation gained the right to reopen an old mine in the heart of the monarch reserve. Experts believe that if the mine is reopened, it will likely spell the end of this magical place.

    And as if all this wasn't bad enough, the increasing use of herbicides（除草剂） in the American corn belt has led to the decrease of  a plant which is essential to the monarchs' development from egg into butterfly.

    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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    In one of the world's most forbidding environments, Polar bears spend !heir summers roaming (漫步) the Arctic on large pieces of floating ice. They drift(漂移) for hundreds of miles, finding mates, hunting for seals , and fatting themselves up for the winter. Without the floating ice , the world's largest bears could not survive Yet at this moment , the polar bears' Arctic habitat (栖息地) is melting away under it because of global warming.

    Over the past three decades, more than a million square miles of floating ice has disappeared Scientists predict that, if the current rate of global warming continues, most, if not all, of the bears' floating ice will be gone by 2100. As a result, the world's polar bears could face global dying out by the end of this century.

    Already, the ice on the southern edge of the polar bears' habitat is melting about three weeks earlier than in the past. The loss of those key weeks leaves the bears less time to hunt, eat and store up fat. Once the ice melts, the bears must fast for up to eight months on land until the floating ice returns. As a result of early melting, there has been a 14 percent decline in the Hudson Bay polar bear population over the past ten years- a fall clearly caused by global warming.

    Besides, a growing number of polar bears may be drowning (溺水) as they are forced to swim more often, and for longer distances, looking for ice sheets(冰原). According to a report by the US Mincrals Management Service, researchers observed four dead polar bears floating 60miles off Alaska in September of2006 and said it was likely that many other bears swimming far off shore also drowned. Scientists have concluded that the worldwide population of about 20,000polar bears is likely to become smaller by more than 30 percent in the next 35 to 50 years alone.

    There is an English saying that “Laughter is the best medicine.” Until recently, few people took the saying seriously. Now, however, doctors have begun to look into laughter and the effects it has on the human body. They have found that laughter can really improve people's health.

    Tests were carried out to study the effects of laughter on the body. People watched funny films while doctors checked their hearts, blood pressure, breathing and muscles. It was found that laughter has similar effects to physical exercise. It increases blood pressure, makes the heart beat quicker and makes people breathe deeper; it also works on several groups of muscles in the face, the stomach, and even the feet. If laughter exercises the body, it must be beneficial(有益的).

Other tests have shown that laughter appears to be able to reduce the effects of pain on the body. In one experiment doctors produced pain in groups of students who listened to different radio programs. The group that tolerated（忍耐）the pain for the longest time was the group which listened to a funny program. The reason why laughter can reduce pain seems to be that it helps to produce a kind of chemical in the brain which diminishes both stress and pain.

    As a result of these discoveries, some doctors in the United States now hold laughter clinics(诊所), in which they help to improve their patients' condition by encouraging them to laugh. They have found that even if their patients do not really feel like laughing, making them smile is enough to produce beneficial effects similar to those caused by laughter.

    A small robot may help children who are recovering from long-term illnesses in the hospital or at home. These children may feel isolated from their friends and classmates. The robot takes their place at school. Through the robot, the children can hear their teachers and friends. They also can take part in class from wherever they are recovering.

    Anyone who has a long-term illness knows that recovering at home can be lonely. This can be especially true for children. They may feel left out. Now, these children may have a high-tech friend to help feel less lonely. That friend is a robot. The robot is called AV1. AV1 goes to school for a child who is homebound while recovering from a long-term illness. And the child's school friends must help. They carry the robot between classes and place the robot on the child's desk.

    A Norwegian company called No Isolation created the robot. The co-founders of No Isolation are Karen Dolva and Marius Aabel. Dolva explains how the robot AV1 works. She says, from home, the child uses an iPad or a phone to start the robot. Then they use the same device to control the robot's movements. At school, the robot becomes the eyes, ears and voice of the child.

    So, it sits at the child's desk in the classroom and the child uses an iPad or a phone to start it, control its movement with touch, and talk through it.

    The student can take part in classroom activities from wherever they are recovering — whether at home or from a hospital bed. The robot is equipped with speakers, microphones and cameras that make communication easy.

Oxford English Dictionary (OED) announced on November 23 for the first time that it has chosen not to name one single word of the year, but many words for the "special" year 2020. Describing 2020 as "a year which cannot be neatly summarized in one single word". OED said on Monday that there were too many words to sum up the events of 2020. From more than 11 billion words found in web-based news, blogs and other text sources, its lexicographers(词典编纂者)revealed what the dictionary described as "great shifts in language data and frequency rises in new words" over the past 12 months.

Most words of the year are coronavirus-related, including coronavirus, lockdown, circuit-breaker, keyworkers and face masks. The report said the word "coronavirus" dates back to the 1960s and was previously "mainly used by scientific and medical specialists". But by April this year it had become one of the most frequently used nouns in the English language, beyond even the usage of the word "time". It said use of the word "pandemic" has increased by more than 57,000 percent this year.

The revolution in working habits during the pandemic has also affected language, with both "remote" and "remotely" seeing more than 300 percent growth in use since March. "On mute(静音)"and "unmute" have seen 500 percent rises since March, while the words "workation" and "staycation" also increased drastically.

Casper Grathwohl, the president of Oxford Dictionaries, said. "I've never witnessed a year in language like the one we've just had. The Oxford team was identifying hundreds of significant new words and usages as the year unfolded." "2020 has been filled with new words unlike any other," Grathwohl added.

The OED's announcement mirrored the huge influence of the COVID-19 on the people from all walks of life. To bring life back to normal, countries such as Britain, China, Germany, Russia and the United States are racing against time to develop coronavirus vaccines.