If you see a group of people dancing and singing on the street or in the railway station, you don't need to feel surprised. They are a flash mob(快闪族), which is a group of people who come together suddenly in a public place, do something unusual for a brief period of time, and then quickly break up. They are usually organized with the help of the Internet or other digital communication network. At a predetermined(预先决定的) time, they gather and perform some distractions(消遣) such as waving their hands and exchanging books. Then, they quickly break up before the police can arrive. Using mobile phones, the flash mob can change its location if the first one has been replaced for any reason.

    Bill Lasik, senior editor of Harper's Magazine, organized the first flash mob in Manhattan in May 2003 and the first successful flash mob came together on June 3, 2003 —after the first attempt failed at Macy's department store. Lasik claimed that the activity was designed to make fun of hipsters(赶时髦的人), and call attention to the cultural atmosphere.

    Flash mob gatherings can sometimes shock people. Such an activity might seem amusing and untrue, but it also might frighten people who are not aware of what is taking place. Undoubtedly, flash mobs can serve as good political tools in any direction. They also have great economic potential, such as using flash mobs to advertise a product.

    The flash mob is now becoming more and more popular. People use it to do many things. For example, in 2009, Michael Jackson's fans took part in a flash mob to remember him. Hundreds of his fans gathered singing and dancing Michael's famous song “Beat It” together. Flash mobs give people from all walks of life an opportunity to come together to create a memory.

    Grown-ups are often surprised by how well they remember something they learned as children but have never practiced ever since. A man who has not had a chance to go swimming for years can still swim as well as ever when he gets back in the water. He can get on a bicycle after many years and still ride away. He can play catch and hit a ball as well as his son. A mother who has not thought about the words for years can teach her daughter the poem that begins “Twinkle, twinkle, little star” or remember the story of Cinderella or Goldilocks and the Three Bears.

    One explanation is the law of overlearning, which can be stated as follows: Once we have learned something, additional learning experiences increase the length of time we will remember it.

    In childhood we usually continue to practice such skills as swimming, bicycle riding, and playing baseball long after we have learned them. We continue to listen to and remind ourselves of words such as “Twinkle, twinkle, little star” and childhood tales such as Cinderella and Goldilocks. We not only learn but overlearn.

    The multiplication tables (乘法口诀表) are an exception to the general rule that we forget rather quickly the things that we learn in school, because they are another of the things we overlearn in childhood.

    The law of overlearning explains why cramming (突击学习) for an examination, though it may result in a passing grade, is not a satisfactory way to learn a college course. By cramming, a student may learn the subject well enough to get by on the examination, but he is likely soon to forget almost everything he learned. A little overlearning, on the other hand, is really necessary for one's future development.

    A scientist once said: "I have concluded that the earth is being visited by intelligently controlled vehicles from outer space."

    If we take this as a reasonable explanation for UFOs (unidentified flying objects), questions immediately come up.

    "Why don't they get in touch with us, then? Why don't they land right on the White House lawn and declare themselves?" people asked.

    In reply, scientists say that, while this may be what we want, it may not necessarily be what they want.

    "The most likely explanation, it seems to me," said Dr. Mead, "is that they are simply watching what we are up to—that responsible society outside our solar system is keeping an eye on us to see that we don't cause a chain reaction that might have unexpected effects for the outside of our solar system."

    Opinions from other scientists might go like this: "Why should they want to get in touch with us? We may feel we're more important than we really are! They may want to observe us only and not interfere(干涉) with the development of our civilization  They may not care if we see them but they also may not care to say 'hello'."

    Some scientists have also suggested that Earth is a kind of zoo or wildlife reserve. Just as we set aside wilderness areas and wildlife reserves to allow animals and growing things to develop naturally while we observe them, so perhaps Earth was set aside ages ago for the same purpose.

    Are we being observed by intelligent beings from other civilizations in the universe? Are they watching our progress in space travel? Do we live in a huge "zoo" observed by our "keepers," but having no communication with them?

    Never before in our history have we had to face ideas bravely like these. The simple fact is that we, who have always regarded ourselves as supreme in the universe, may not be so. Now we have to recognize that, among the stars in the heavens, there may very well be worlds lived by beings who are to us as we are to ants.

    It is a question people have been asking for ages—is there a way to turn back the aging process?

    For centuries, people have been looking for a "fountain of youth." The idea is that if you find a magical fountain, and drink from its waters, you will not become old. Spanish explorer Juan Ponce de León searched for waters with magical powers in the early 1500s. But what he found instead is the American State of Florida.

    Researchers in New York did not find a fountain of youth in fact, but they may have found a way to turn back the aging process. It appears the answer may be hidden right between your eyes, in an area called the hypothalamus (丘脑下部).The hypothalamus is part of your brain. It controls important activities within the body, including growth, reproduction and the way we process food.

    Researchers at New York's Albert Einstein College of Medicine found that hypothalamus stem cells also influence how fast aging takes place in the body. What are stem cells? They are simple cells that can develop into specialized cells, like blood or skin cells. Stem cells can also repair damaged parts in the body.

    Professor Dongsheng Cai was the lead researcher in a study on aging in mice. Cai explains in the Journal Nature what they found. "Aging speed can be controlled by a particular place in the body, which is the hypothalamus. And it can be controlled by a particular type of cells, which are hypothalamus stem cells. I think these findings are quite interesting."

    He adds that when the hypothalamus starts aging, so does the body. "So when hypothalamus function is decreasing, particularly the loss of hypothalamus stem cells, this protection against the aging development is lost. It finally leads to aging."

    Using this information, the researchers began trying to activate (激活) the hypothalamus in mice. They did this by putting stem cells from younger mice into middle-aged animals. Later, the researchers examined mice and tested for changes in behavior. They also looked for changes in the strength of the animals' muscles and the way they worked. The researchers say the results show that the treatment slowed aging in the animals.

    Cai says when hypothalamus stem cells, which were derived from young mice was put into middle-aged mice, the mice aged slowly and they also have increased the length of their lives.

    But these results were just from studying mice in a lab. If the mice can live longer, does that mean people could have longer lives? The next step is to see if the anti-aging effects also work in human beings.

    In its early history, Chicago had floods frequently, especially in the spring, making the streets so muddy that people, horses, and carts got stuck. An old joke that was popular at the time went something like this: A man is stuck up to his waist in a muddy Chicago street. Asked if he needs help, he replies, "No, thanks. I've got a good horse under me."

    The city planner decided to build an underground drainage （排水） system, but there simply wasn't enough difference between the height of the ground level and the water level. The only two options were to lower the Chicago River or raise the city.

    An engineer named Ellis Chesbrough convinced the city that it had no choice but to build the pipes above ground and then cover them with dirt. This raised the level of the city's streets by as much as 12feet.

    This of course created a new problem: dirt practically buried the first floors of every building in Chicago. Building owners were faced with a choice: either change the first floors of their buildings into basements, and the second stories into main floors, or hoist the entire buildings to meet the new street level. Small wood-frame buildings could be lifted fairly easily. But what about large, heavy structures like Tremont Hotel, which was a six-story brick building?

    That's where George Pullman came in. He had developed some house-moving skills successfully. To lift a big structure like the Tremont Hotel, Pullman would place thousands of jackscrews (螺旋千斤顶) beneath the building's foundation. One man was assigned to operate each section of roughly 10jackscrews.At Pullman's signal each man tured his jackscrew the same amount at the same time, thereby raising the building slowly and evenly. Astonishingly, the Tremont Hotel stay open during the entire operation, and many of its guests didn't even notice anything was happening.

    Some people like to say that every problem has a solution. But in Chicago's early history, every engineering solution seemed to create a new problem. Now that Chicago's waste water was draining efficiently into the Chicago River, the city's next step was to clean the polluted river.

PRACTITIONERS

Jacqueline Felice de Almania(c.1322) highlights the suspicion that women practicing medicine faced.Born to a Jewish family in Florence,she moved to Paris where she worked as a physician and performed surgery.In 1322 she was tried for practicing unlawfully.In spite of the court hearing testimonials(证明)of her ability as a doctor,she was banned from medicine.

Tan Yunxian(1461-1554) was a Chinese physician who learned her skills from her grandparents.Chinese women at the time could not serve apprenticeships(学徒期) with doctors.However,Tan passed the official exam.Tan treated women from all walks of life.In 1511,Tan wrote a book,Sayings of a Female Doctor,describing her life as a physician.

James Barry(c.1789-1865) was born Margaret Bulkley in Ireland but,dressed as a man,she was accepted by Edinburgh University to study medicine.She qualified as a surgeon in 1813,and then joined the British Army,serving overseas.Barry retired in 1859,having practiced her entire medical profession living and working as a man.

Rebecca Lee Crumpler(1831-1895) worked as a nurse for eight years before studying in medical college in Boston in 1860.Four years later,she was the first African American woman to receive a medical degree.She moved to Virginia in 1865,where she provided medical care to freed slaves.