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.

    Now many young people are traveling around the world on their own，not because they have no one to travel with，but because they prefer to go alone.

    Kristina Wegscheider from California first traveled alone when she was at college and believes that it is something everyone should do at least once in their life. ＂It opens up your mind to new things and pushes you out of your comfort zone.＂ Wegscheider has visited 46 countries covering all seven continents.

    In foreign countries，with no one to help you read a map，look after you if you get ill，or lend you money if your wallet is stolen，it is challenging. This is what drives young people to travel alone. It is seen as character building and a chance to prove that they can make it on their own.

    Chris Richardson decided to leave his sales job in Australia to go traveling last year. He set up a website，The Aussie Nomad，to document his adventures. He says he wished he had traveled alone earlier. ＂The people you meet，the places you visit，or the things you do，everything is up to you and it forces you to grow as a person，＂ said the 30-year-old man.

    Richardson describes traveling alone like ＂a shot in the arm＂，which ＂makes you a more confident person that is ready to deal with anything＂. He said，＂The feeling of having overcome something on my own is a major part of what drives me each day when I'm dealing with a difficult task. I walk around with my head up because I know deep down inside that nothing is impossible if you try.＂

    The great 19th century explorer John Muir once said，＂Only by going alone in silence can one truly get into the heart of the wilderness.＂

It's a popular belief that a fish's memory lasts for only seven seconds. It may seem sad to think that they don't remember what they've eaten or where they've been, and they don't recognize you or any of their friends—every moment in their life would be like seeing the world for the first time.

    But don't be so quick to feel sorry for them. A new study has found that fish have a much better memory than we used to think. In fact, certain species of fish can even remember events from as long as 12 days ago.

    In the study, researchers from MacEwan University in Canada trained a kind of fish called African cichlids to go to a certain area of their tank to get food. They then waited for 12 days before putting them back in the tank again.

Researchers used computer software to monitor the fish's movements. They found that after such a long break the fish still went to the same place where they first got food. This suggested that they could remember their past experiences.

    In fact, scientists had been thinking for a long time that African cichlids might have a good memory. An earlier study showed that they behaved aggressively in front of certain fish, perhaps because they remembered their past “fights”. But until the latest findings, there was no clear evidence.

    Just as a good memory can make our lives easier, it also plays an important part when a fish is trying to survive in the wild.

    “If fish are able to remember that a certain area contains safe food, they will be able to go back to that area without putting their lives at risks,” lead researcher Trevor Hamilton told Live Science.

    For a long time, fish were placed far below chimpanzees, dolphins and mice on the list of smart animals. But this study has given scientists a new understanding of their intelligence.

    We're often reminded of the importance of preserving the planet as we see it for future generations—and children at St Oswald's Primary School in Chester certainly agree.

    Nine-year-old Isobel Kelleher from the school's Hummingbirds class thinks adults need to take note. “Sometimes they can be busy and I don't think they think they can make a difference，but if everyone does a little bit it all adds up，” she tells HuffPost UK.“ We started looking at plastic pollution in our oceans and the things like plastic bags and broken down pieces of plastic that are polluting them.” she says.“ Fish can eat the plastic and they can die，or we might even eat the fish ourselves.”

    Mr Timms，Isobel's teacher，has been spearheading a new project at the school which lets children loose creatively to raise awareness of the need to be more environmentally friendly. The entire Hummingbirds class，which is made up of 9 and 10-year-old pupils，has been busy writing poems and creating online video adverts to warn adults about the serious situation of our oceans and wildlife.

    Mr Timms thinks children have an important role to play in teaching us how to take care of the things around us. We sometimes overlook how much we can really learn from children.” he says.“ It has been really hard to believe having parents come in saying that their children have been asking them to stop using plastic，and to recycle more，and even stopping them using plastic straws.”

    Mr Timms is proud of his Hummingbirds class.“ The message that they would like to send to the world is simple stopping this isn't someone else's job，and it won't be OK if we just leave it.”

    No one knows how much money Dr. Lyle Tullis gave away to students. I was his colleague for nearly a decade and I never stopped being amazed at his generosity. Our college has a program of providing cross-cultural experiences for students. Lots of students take advantage of summer experiences oversea. I discovered that no group left for overseas without some of its members receiving financial help from Dr. Tullis.

    It wasn't that he made a lot of money. For one thing, he taught in a church-run school. There, his salary (薪水) was half of what those people earned in a tax-supported school.

    Other colleagues occasionally complained about the low pay. Not Lyle Tullis. Occasionally some professors would leave our campus for a better-paid position. They told me they did so because, with better pay, they could provide for their families.

    The size of Lyle Tullis' paycheck never seemed to be the most important thing to him. I realized that one day when I was thanking him for helping a student, his eyes shone as he said to me, "I've got so much money that I don't know what to do with it. So, I just give it away."

    Most people wouldn't have thought that way. Dr. Tullis drove one of the oldest cars on campus. It was even older than almost any of the students' cars. His home, while comfortable, was not fancy at all. But Lyle Tullis lived with the feeling that he had so much money that he needed to give it away.

    He was one of the favorite on campus. Cynics (愤世嫉俗) might say he bought fame. But they would misunderstand. Lyle wanted to devote his life, all of it, to helping others.

    A few weeks ago, I called an Uber to take me to the Boston airport for a flight home for the holidays. As I slid into the back seat of the car, the warm intonations(语调) of the driver's accent washed over me in a familiar way.

    I learned that he was a recent West African immigrant with a few young children, working hard to provide for his family. I could relate: I am the daughter of two Ethiopian immigrants who made their share of sacrifices to ensure my success. I told him I was on a college break and headed home to visit my parents. That's how he found out I went to Harvard. An approving eye glinted at me in the rearview window, and quickly, we crossed the boundaries of rider and driver. I became his daughter, all grown up—the product of his sacrifice.

    And then came the fateful question: "What do you study?" I answered "history and literature" and the pride in his voice faded, as I knew it might. I didn't even get to add "and African-American studies" before he cut in, his voice thick with disappointment, "All that work to get into Harvard, and you study history?"

    Here I was, his daughter, wasting the biggest opportunity of her life. He went on to deliver the age-old lecture that all immigrant kids know. We are to become doctors (or lawyers, if our parents are being generous)—to make money and send money back home. The unspoken demand, made across generations, which my Uber driver laid out plainly, is simple: Fulfill your role in the narrative(故事) of upward mobility so your children can do the same.

    I used to feel anxious and backed into a corner by the questioning, but now as a junior in college, I'm grateful for their support more than anything. This holiday season, I've promised myself I won't get annoyed at their inquiries. I won't defensively respond with "but I plan to go to law school!" when I get unrequested advice. I'll just smile and nod, and enjoy the warmth of the occasion.