Mark Twain, the famous American writer and a great master of humor, liked to play jokes on others. But once a joke was played on him. One day Mark Twain was invited to give a talk in a small town. At lunch he met a young man, one of his friends.

    The young man said that he had an uncle with him. He told Mark Twain that his uncle never laughed or smiled, and that nobody and nothing was able to make his uncle smile or laugh.

    "You bring your uncle to my talk this evening," said Mark Twain. "I'm sure I can make him laugh."

    That evening the young man and his uncle sat in the front. Mark Twain began to speak. He told several funny stories and made everyone in the room laugh. But the man never even smiled. Mark Twain told more funny stories, but the old man still kept quiet. Mark Twain continued to tell his funny stories. Finally he stopped. He was tired and quite disappointed.

    Some days later, Mark Twain told another friend what had happened." Oh," said his friend, "I know that man. He's been deaf for years."

    On Monday, the Centers for Disease Control and Prevention predicted that 42% of Americans could be overweight by 2030. Our expanding waistlines lead to not only a medical problem, but according to a recent article In The New York Times, it could also endanger personal safety in some situations—in an airplane crash, for example.

    The New York Times' Christen Negroni reports that engineers and scientists are questioning whether airplane seats are adequately constructed to protect overweight travelers. Government standards for airplane seat strength（强度）— first set moe than 60 years ago — require that the seats be made for a passenger weighing 170 pounds (77kg). Today, the average American man weighs nearly 194 pounds (88kg) and the average woman 165 pounds (75kg). Negroni reports:

    “If a heavier person completely fills seat, the seat is not likely to behave as intended during a crash,” said Robert Salazar, the leading scientist at the Center for Applied Biomechanics at the University of Virginia. “The energy absorption that is built into the aircraft seat is likely to be overpowered and the passengers will not be protected properly.”

    “Nor would the injury be limited to that passenger only,” Dr. Salzar said. “If a seat or a seat belt fails,” he said, “those people who are seated nearby could be endangered from ‘the uncontrolled movements of the passenger'.”

    Most complaints about airplane seats focus on their lack of comfort and high ticket price, and whether overweight passengers should be made to buy two seats. But The New York Times' article brings up another reason to feel anxious about flying. Investigators of the issue got in touch with the airplane seat and seat belt makers, but they refused to comment on the problem. Experts agreed that crash testing should be done with overweight dummies （人体模型）. Both airplane seats and seat belts should be tested, they said.

    Fortunately, however, according to Nora Marshall, a senior adviser at the National Transportation Safety Board, the board's investigators have never seen an accident involving a commercial plane in which the weight of a passenger was a problem.

    A new study of 8,000 young people in the Journal of Health and Social Behavior shows that although love can make adults live healthily and happily, it is a bad thing for young people．Puppy love（早恋）may bring stress for young people and can lead to depression (沮丧，抑郁）. The study shows that girls become more depressed than boys, and younger girls are the worst of all．

    The possible reason for the connection between love and higher risk of depression for girls is “loss of self”．According to the study，even though boys would say “lose themselves in a romantic relationship”，this “loss of self” is much more likely to lead to depression when it happens to girls. Young girls who have romantic relationships usually like hiding their feelings and opinions．They won't tell that to their parents．

    Dr Marianm Kaufman，an expert on young people problems，says 15% to 20% young people will have depression during their growing. Trying romance often causes the depression．She advises kids not to jump into romance too early. During growing up，it is important for young people to build strong friendships and a strong sense of self．She also suggests the parents should encourage their kids to keep close to their friends, attend more interesting school activities and spend enough time with family．

    Parents should watch for signs of depression—eating or mood changes—and if they see signs from their daughters or sons, they need to give help. The good news is that the connection between romance and depression seems to become weak with age．Love will always make us feel young，but only maturity （成熟） gives us a chance to avoid its bad side effects．

    When another old cave is discovered in the south of France, it is not usually news. Rather, it is an ordinary event. Such discoveries are so frequent these days that hardly anybody pays heed to them. However, when the Lascaux cave complex was discovered in 1940, the world was amazed. Painted directly on its walls were hundreds of scenes showing how people lived thousands of years ago. The scenes show people hunting animals, such as bison or wild cats. Other images depict birds and, most noticeably, horses, which appear in more than 300 wall images, by far outnumbering all other animals.

    Early artists drawing these animals achieved a monumental and difficult task. They did not limit themselves to the easily accessible walls but carried their painting materials to spaces that required climbing steep walls or going on hands and knees into narrow passages in the Lascaux complex.

    Unfortunately, the paintings have been exposed to the destructive action of water and temperature changes, which easily wear the images away. Because the Lascaux caves have many entrances, air movement has also damaged the images inside.

    Although they are not out in the open air, where natural light would have destroyed them long ago, many of the images have deteriorated and are hardly recognizable. To prevent further damage, the site was closed to tourists in 1963, 23 years after it was discovered.

    If you have spent any time living in or visiting a big city in China, you have most likely used the service that Didi provides. The convenient ride-sharing company, now known as Didi Chuxing, was praised last month by Fortune magazine. According to the magazine, it is changing the world with its environmentally friendly solution to getting around in major cities. As the magazine reported: “Didi concluded that last year its car-pooling services helped reduce total car trips in the country by 1 million a day, which saved 500 million liters of gas, cutting 13.5 million tons of carbon emissions (排放) per day.” The transport service was the only Chinese company to make the important magazine's list, and its influence can be felt in many parts of Chinese society.

    Didi is just one example of what is being called the “sharing economy”. The sharing economy is one in which regular people exchange goods and services, usually using an online marketplace. Other similar companies include companies that allow people to rent out their homes to strangers like Airbnb and Couchsurfing, and similar ride sharing services like Uber and Lyft.

    All of these services are possible partly because many people are realizing that they have underused assets in their lives. Millions of people around the world have started renting them out online to earn extra money. They rent their apartments while they are away for the weekend, lend people their cars for money and even sell their spare time.

    The sharing economy is the latest example of the Internet's value to customers. This model is now big enough for regulators and companies to have woken up to it. That is a sign of its great potential. It is time to start caring about sharing.

    Helen Keller was one of America's best-known women. She was admired for her courage and achievements although she couldn't see or hear. She was also known throughout the world for herself-sacrificing work to improve the condition of the blind, the deaf and the speechless. When she died on June 1, 1968, the newspaper Washington Post wrote: "Her life was truly one of the most remark able phenomena of our time and her death just short of the age of 88 leaves the whole world poorer."

    Helen Keller was born on June 27th, 1880, in Tuscumbia, Alabama. For the first 19 months of her life, she was a pretty and happy baby, normal in every way. Then a sudden illness destroyed her sight and her hearing. Because she could not hear sounds to imitate(模仿), she could not speak. Helen used to say that her real birthday was not June 27th, 1880, but March 3rd, 1887—the day when Anne Sullivan entered her life. It was Anne Sullivan who taught Helen to spell certain words by a special system, Braille, and even to talk.

    Anne Sullivan could not teach Helen Keller to speak until some other important things had been learned. The little girl had to learn to control her actions and feelings. She had to learn that she could not always do what she wanted to do. She had always been able to get what she wanted by using force. The teacher had to change such habits without breaking the child's spirit.

    Miss Sullivan's battle began. Sometimes, there was real fighting between the wild child and the strong young teacher. At last, however, the battle was won by Miss Sullivan, who succeeded in showing Helen that she loved her and wanted to help her. The child and her teacher became friends. They continued to be friends until the teacher's death, fifty years later.

    The day on which Helen finally accepted Miss Sullivan as her friend and teacher was a great day in Helen's life. After that, the teacher could begin to teach the child language.