Below is a selection from a popular science book.

    If blood is red, why are veins（静脉） blue?

    Actually, veins are not blue at all. They are more of a clear, yellowish color. Although blood looks red when it's outside the body, when it's sitting in the vein near the surface of the skin, it's more of a dark reddish purple color. At the right depth, these blood-filled veins reflect less red light than the surrounding skin, making them look blue by comparison.

    Which works harder, you heart or your brain?

    That kind of depends on whether you're busy thinking or busy exercising. Your heart works up to three times harder during exercise, and shifts enough blood over a lifetime to fill a supertanker. But in the long run, your brain probably tips it, because even when you're sitting still your brain is using twice as much energy as your heart, and it takes four to five times as much blood to feed it.

    Why do teeth fall out, and why don't they grow back in grown-ups?

    Baby (or “milk”) teeth do not last long; they fall out to make bigger room for bigger, stronger adult teeth later on. Adult teeth fall our when they become damaged, decayed and infected by bacteria. Once this second set of teeth has grown in, you're done. When they're gone, they are gone. This is because nature figures you're set for life, and what controls regrowth of your teeth switches off.

    Do old people shrink as they age?

Yes and no. Many people do get shorter as they age. But, when they do, it isn't because they're shrinking all over. They lose height as their spine（脊柱） becomes shorter and more curved due to disuse and the effect of gravity(重力). Many (but not all) men and women do lose height as they get older. Men lose and average of 3-4 cm in height as they age, while women may lose 5 cm or more. If you live to be 200 years old, would you keep shrinking till you were, like 60 cm tall, like a little boy again? No, because old people don't really shrink! It is not that they are growing backwards —- their legs, arms and backbones getting shorter. When they do get shorter, it's because the spine has shortened a little. Or, more often, become more bent and curved.

    Why does spinning make you dizzy（眩晕的）?

    Because your brain gets confused between what you're seeing and what you're feeling. The brain senses that you're spinning using special gravity-and-motion-sensing organs in your inner ear, which work together with your eyes to keep your vision balance stable. But when you suddenly stop spinning the system goes out of control, and your brain thinks you'removing while you're not.

    Where do feelings and emotions come from?

Mostly from an ancient part of the brain called the limbic system. All mammals have this brain area —- from mice to dogs, cats, and humans. So all mammals feel basic emotions like fear, pain and pleasure. But since human feelings also involve other, newer bits of the brain, we feel more complex emotions than any other animal on this planet.

    If exercise wears you out, how can it be good for you?

    Because our bodies adapt to everything we do to them. And as far as your body is concerned, it's “use it, or lose it”! It's not that exercise makes you healthy, it's more that a lack of exercise leaves your body weak and easily affected by disease

    “Patience” is a word that you have probably heard a lot! But you may wonder, “What on earth is patience, and why does everyone keep telling me to have it?”

    Patience is waiting for something or someone. It is accepting delay without getting angry or upset.

    ※It is waiting for someone else to speak when you have something to say !

    ※It is waiting to eat before everyone is at table !

    ※It is waiting for your birthday even though you may want a present now !

    ※Patience is trying something again when you want to give up.

    Patience doesn't sound like much fun. And to be truthful, sometimes, it's not. And with so much technology in the world, we don't always have to be patient. For example, we are able to stream our favorite TV shows on our cell phones, go to restaurants where food is served hot and fast, among many other things.

    So, why wait when we can have everything right now? Because just about any successful person has patience. Many good and important things take patience. For example, most great inventors and explorers have a lot of patience!

    ※When Thomas Edison invented the light bulb, it took him over 1,000 tries!

※It took the Wright Brothers years to invent the airplane!

    ※It takes between 150 and 300 days to travel from Earth to Mars!

    ※If great inventors did not have patience and self-control, you would not be reading this right now. In fact, you might be living in a cave or tent somewhere. You would have no electricity, no car, no school and almost certainly no Internet!

    To eat only a vegan（素食的） diet might sound difficult, especially with many popular dishes containing meat or other animal products. Yet in 2017, veganism was described as “the fastest - growing lifestyle movement” in the UK, according to BBC news.

    A study by a UK market research organization, suggests that over 542,000 Brits went vegan during the previous decade, an increase of 360%. And the main force behind this increase was those aged 15-34 years old — 42% of recent vegans fall under that age range.

    So why exactly has veganism become so popular among young Brits?

    One reason could be that many young people want to protect the environment, as less meat consumption is known to be beneficial to the environment. Global meat manufacturing is believed to cause 18% of the world's greenhouse gas emissions（排放）, even more than that of all the world's cars, trains and planes combined. Another reason is that many of today's young people believe that it's wrong to kill animals to be used as food.

    Jess Murray, 22, a student at University College London, said that he chose to become vegan after realizing that eating animals is a choice that people make, rather than something that we need to do to survive. “Becoming vegan was an ethical (道德的) decision,” he told the Guardian.

    Social media is also believed to have given rise to the increase in veganism. Platforms such as Facebook allow young vegans to connect with each other much easier, while others such as Instagram have led to the creation of “vegan celebrities (名人)”, who share lips on vegan lifestyles.

    Despite the rising of veganism, Laura Wyness, an expert in diet and nutrition, said that meat is very important for people's health. “A strict vegan diet makes it difficult to get some minerals and vitamins that your body needs,” Wyness told BBC news.

    However, the popularity of veganism doesn't seem to be fading. “It feels more like this is something that is sticking.” said an expert of the Vegan Society, a UK charity that promotes veganism.

    It is hard to say the first day of school in the United States because when the first day of school is and what happens on the first day of school usually are different by districts(地区).

    The first day of school for many school districts in different states is on the day after the first Monday in September. In some other school districts, school begins in mid-to-late August. For example, the Denver, Colorado schools go back in mid-August and schools in Cleveland, Ohio start back usually one week before the first Monday in September. The Boston, Chicago, New York City and San Diego schools start back on the Tuesday or Wednesday after the first Monday in September.

    Schools in Cleveland, Ohio used to start the school year on the day after the first Monday in September, but in the 1976-1977 and 1977-1978 academic years, the school year was affected by several bad snowstorms, extreme cold. In the 1978-1979 school year, the Ohio Department of Education moved the start of the year to late August, one week before the first Monday in September. This went into effect in the 1980-1981 school year.

    In most school districts in Utah, the school year starts between August 25 and August 30, and goes until the last week of May or the first week in June next year.

    In American high schools, the freshmen class usually goes back one or two days before the rest of the school body for an orientation(迎新)period. An orientation period helps the freshmen get familiar with their new school, its rules, and surroundings.

    In some schools, the freshmen classes have their photographs taken for identification purposes. Some high schools have tried to make the first week of school fun for incoming freshmen.

Natural selection is the process by which one type of animal within a species develops well because of certain characteristics that make it more likely to live than others in its group. The history of the peppered moth(桦尺蛾) is an example of the natural selection process.

In nineteenth-century England, certain types of peppered moths were able to better blend (融合) into their surroundings. During that time period, great changes were happening in Great Britain. The Industrial Revolution was part of this change, and with it came air pollution. Natural selection usually takes hundreds or even thousands of years to occur. For the peppered moth, this process occurred relatively quickly .

At the beginning of the Industrial Age, most peppered moths in England were light-coloured and covered with black markings, although a few moths had dark-coloured wings. Because the light-coloured moths blended into the light-coloured bar k on the trees, they could not be easily seen by birds that would eat them. As the air grew more polluted, however, tree trunks (树干) became covered with soot (煤烟) and became darker. The light-coloured moths became easy for birds to see against the dark tree trunks. Since the dark-coloured moths now had the advantage, their numbers grew. Within 50 years, the peppered moth went from being mostly light-coloured to being mostly dark-coloured.

In the twentieth century, the air cleared up, and the peppered moth population changed again. As tree trunks lightened due to less soot in the air, light-coloured moths once again had an advantage. Their numbers increased as soot levels declined. Depending on their environment, the colouration of the moths helped them to be "naturally selected" to survive.