For centuries, people wondered how big the Earth was. Unfortunately, although the Earth was flat, no one was able to figure out its size. Gradually, however, people began to realize that the Earth was really round.

    Then, in the third century BC(2,300 years ago), a Greek man named Eratosthenes had an idea. Eratosthenes was sure that the Earth was a sphere. He used the sun and geometry(几何学) to figure out the size of the Earth. He calculated that the circumference(圆周) of the Earth was 28,600 miles(46,000 kilometers). The true size of the Earth is 25,000 miles(40,000 kilometers).

    Eratosthenes' measurement was wrong, but it was very close to the truth.

    For many centuries after Eratoshenes lived, people made maps of the Earth. However, they did not know very much about the world outside of Europe, Asia, and North Africa. Mapmakers could not draw accurate maps of the Earth until people began traveling around the world in the fifteenth century, mapping small areas each time. In the eighteenth and nineteenth centuries, people began making correct maps of countries, but the first accurate maps of the world were not made until the 1890s.

    Maps today are reliable(可依靠的), inexpensive, and easy to understand. People depend on maps every day. What would our lives be like without them?

Building your healthy diet

    Although some extreme diets may suggest something else, we all need a balance of protein ( 蛋白质), fat, carbohydrates ( 碳水化合物), fiber, vitamins, and minerals in our diets to keep a healthy body. You don't need to remove certain kinds of foods from your diet, but rather select the healthiest ones.

Protein

    Protein gives us the energy to get up and go — and keep going — while also supporting mood and cognitive function. Too much protein can be harmful to people with kidney  ( 肾 )  disease,  but  the  latest  research  suggests  that  many  of  us  need  more high-quality protein, especially as we grow up. That doesn't mean you have to eat more animal products — a variety of plant-based sources of protein each day can ensure your body gets all the essential protein it needs.

Fat

    Not all fat is the same. While bad fat can destroy your diet and increase your risk of certain diseases, good fat protects your brain and heart. In fact, healthy fat — such as omega-3s — is necessary to your physical and emotional health. Understanding how to include more healthy fat in your diet can help improve your mood and your well-being.

Fiber

    Eating foods high in dietary fiber (grains, fruit, vegetables, nuts, and beans) can help you stay regular and lower your risk of heart disease, stroke, and diabetes. It can also improve your skin and even help you to lose weight. Depending on your age and gender, nutrition experts suggest you eat at least 21 to 38 grams of fiber each day for optimal (最 佳的) health. Unfortunately, most of us aren't eating even half that amount.

Calcium (钙)

    Your body uses calcium to build healthy bones and teeth, keep them strong, send messages through the nervous system, and regulate the heart's rhythm. Not getting enough calcium in your diet can also lead to anxiety, depression, and sleep difficulties. Whatever your age or gender (性别), it's necessary to include calcium-rich foods in your diet.

    When I was 19 years old, I was at a dance club. As we were walking to my car one cold night, a man walked up to us. Behind him was a woman carrying a small child. The child had a jacket on but it wasn't buttoned up（扣上）. The man began to tell us he wanted to borrow some money for the night to get his wife and kid into a hotel. He had a job but no place to live in and was waiting for the first paycheck. He said he could get our mailing address and mail the money back.

    The guy I was with reached into his pocket to give this man a $20 bill. As the other man was extending his hand out to take the money, I put my hand on my new friends' hand and said, “Can I talk to you for a minute？"

    I told him that every day people asked my mother for money on her way to work. She said they made more money than she did, simply begging for money. These people were scamming those with soft hearts. And if they were truly worried about their child suffering from the cold, they would have at least buttoned his jacket or covered him with his blanket.

    My new friend looked at me with disappointment and said, “Michelle, I know there are people out there that take advantage of others. I also know there are people out there that are one paycheck away from being homeless. If I give $ 20 to 10 people and only one of them really needs it and uses it for the right thing, it is worth it.”

    I am now 37 years old and have never forgotten what he said to me. I don't even remember his name. But I do remember that that experience changed the way I look at different situations.

    Are you a procrastinator（拖拉者）? It's a common problem. And it can be harmful, as previous research has shown that people who procrastinate have higher levels of stress and lower well-being, reported the Association for Psychological Science in the US. But a new study has found a way to deal with this problem: Be more connected to your future self.

    Psychologists think that each person believes that they are really two people: “Present Me” and “Future Me”. “People act as if they prefer their current selves' needs and desires to those of their future selves,” wrote psychologists Neil Lewis and Daphna Oyserman.

    Oyserman and Lewis decided to try to find a way to make “Present Me” imagine exactly how “Future Me” would feel the night before a big paper was due, though “Present Me” hadn't started yet. They made “Present Me” think about a far-off event as a number of days away, not months or years.

    Thinking about events in this way meant that something like a friend's wedding seemed 16.3 days sooner when considered in days rather than months and 11.4 months sooner when considered in months rather than years.

    The researchers also tried to find out whether people would take action sooner if they were told a certain event was happening in X days rather than years.  For example, participants imagined they had a newborn child, and that the child would need to go to college in either 18 years or 6,570 days. The researchers found those in the “days” condition planned to start saving four times sooner than those in the “years” condition.

    So if you think of your life in days instead of years, you may get things done quicker.

    Here's an idea whose time has come: A flu shot that doesn't require an actual shot.

    For the first time, researchers have tested a flu vaccine patch (疫苗贴) in a human clinical experiment and found that it delivered as much protection as a traditional injection with a needle. Doctors and public health experts have high hopes that it will increase the number of people who get immunized (免疫的) against the flu.

    Seasonal flu is responsible for up to half a million deaths around the world each year according to the World Health Organization. A team led by Georgia Tech engineer Mark Prausnitz has come up with an alternative method that uses “microneedles”. These tiny needles are so small that 100 of them, arranged in order on a patch, can fit under your thumb (拇指). Yet they're big enough to hold vaccine for three types of flu.

    None of the study volunteers had serious side effects. The groups that got patches had mild skin reactions that were not seen in the regular needle group, while the volunteers in the regular needle group were more likely to experience pain. Overall, 70 percent of the volunteers who got vaccine patches said they'd rather use them again than get a traditional flu shot. The study authors declared it a success on all fronts.

    The biggest beneficiaries could be people in low- and middle-income countries, where flu vaccines are hard to come by. Reducing pain is nice, but other benefits—the patch costs less, is easier to transport, doesn't require refrigeration, can be self-administered and doesn't cause waste of needles—are even better.

    "Microneedle Patches have the potential to become ideal candidates for vaccination programs," wrote Katja Hoschler and Maria Zambon of Public Health England.

    Put an ice cube from your fridge into a glass of water. You have a piece of string (线) 10 centimeters long. The problem is to take out that piece of ice with the help of the string. But you must not touch the ice with your fingers.

    You may ask your friends to try to do that when you are having dinner together. There is a saltcellar on the table. You must use salt when you carry out this experiment.

    First you put the string across the piece of ice. Then put some salt on the ice. Salt makes ice melt (融化). The ice round the string will begin to melt. But when it melts, it will lose heat. The cold ice cube will make the salt water freeze again.

    After a minute or two you may raise the piece of string and with it you will raise your piece of ice!

    This experiment can be very useful to you. If, for example, there is ice near the door of your house, you must use very much salt to melt all the ice. If you don't put enough salt, the water will freeze again.