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 trials 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.

    The TED speaker series features “ideas worth spreading”. With over 1,400 to choose from, we've selected a few that are perfect for students.

    Larry Smith: Why you will fail to have a great career

    We humans may have an unfavorable habit of making excuses for ourselves of or being too confident about ourselves. Accordingly, Larry Smith, a professor of economics at the University of Waterloo in Canada, tells us why most of us will fail to have a great career. Luckily, as he puts it, there is a way out-follow your heart, as long as it is good for your career.

    Andy Puddicombe: All it takes is 10 mindful minutes

Between dance team, volunteering and –oh, right—lectures, your life's crazy factor（因素）is about to go way up. In this entertaining and informative talk, Mindfulness, expert Andy Puddicombe teaches us how to be “more healthy, more mindful and less distracted” by taking just 10 minutes out of the day to be “more present”.

    Shane Koyczan: To this day…for the bullied（欺凌）and beautiful

    This talk is sure to stay with you. Shane Koyczan's “To This Day” is an affecting spoken-word poem about bullying and being different that gained over 10 million views on YouTube. In this talk, Koyczan gives a live reading of the poem, along with some stories about his background.

    Susan Cain: The power of introverts（性格内向者）

    Does a cup of tea and a good book sound like a perfect Friday night? In this personal talk, Susan Cain argues that introverts have talents and abilities. Our culture may value being outgoing, but the world needs all kinds.

Yellowstone Weather

    Yellowstone National Park is at high altitude. Most of the park is above 7,500 feet (2,275 meters).

    Yellowstone's weather is unpredictable. In summer, it may be warm and sunny with temperatures in the high 70s. At night in any given month, the temperature may drop close to freezing. So it is best to come prepared for cold evenings and mornings, especially if you are camping or hiking. When you leave your campsite, please leave it prepared for possible thundershowers and wind.

    A sunny warm day may become fiercely stormy with wind, rain, sleet and sometimes snow. Without enough clothing, and easy day hike or boat trip can turn into a battle for survival.

    There was a doctor in our city who was famous for his skills. Once he received a little child who was badly ill. Thanks to his skill and care, his young patient got well and was soon able to get up and run about again.

    The child's mother was very much thankful to the doctor, and she visited him to thank him for what he had done for her child. “Doctor,” she said, “you have saved my little son. I don't know how to thank you enough. I feel that money alone cannot repay you, so I have made this little purse with my own hands, as a sign of my gratitude. I hope you will accept it.” The doctor stood up and said coldly, “Madam, a little present like that is very nice between friends, but a doctor needs to be paid properly for what he has done.” The lady was so surprised and hurt so much that she could not reply for a moment. Then she said quietly, “Perhaps you will tell me how much your fee is?”

    “Fifty pounds”, he answered.

    The lady opened the little purse and took out four fifty-pound bank notes. She handed one of them to the doctor, and put the other three back into the purse. She put the purse into her handbag and, saying good bye to the doctor, went out of the room.

    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.