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

    Earlier this month, blogger Lisa Henderson announced that she and her husband John had decided not to have Christmas. The family, who lives in Utah, will still put up decorations, but presents from Santa are a no-go this year.

    “John and I feel like we are fighting a very hard uphill battle with our kids when it comes to their rights,” Henderson wrote on her blog. “It is one of the biggest struggles as a parent these days in middle class America. Our kids have been acting so ungrateful lately. ... John said, “We shouldn't just celebrate Christmas. And, so that's what we did.”

    Instead, the Hendersons are putting the money they would have spent on gifts toward service projects in order to teach their three sons the “pleasure of giving.” The children will still receive gifts from grandparents and other family members, but this year, she said, their letters to Santa will be asking him to find someone who needs presents more than they do.

    In an interview with ABC News, 11-year-old Caleb Henderson admitted that he and his brothers had been behaving badly. “We would hit each other. We were fighting and crying,” he admitted, and Lisa said that when she broke the news to her sons, they cried pretty hard.

    But so far, Henderson told Fox News last week, the family is having a sudden turning this into a different kind of gift. They have already held a clothing drive and sent boxes of clothes and candy to a village in the Philippines that was hit hard last year by Typhoon Haiyan.

    “The children were excited and kept wanting to give more and more,” she reported on her blog.

    Many readers responded positively to Henderson's post. Some sharing their own stories of limiting Christmas in order to teach their children to be charitable(仁慈的) or grateful. “As parents you're giving your kids something so much more special than a bunch of gifts on Christmas,” one wrote.

    But Henderson received negative follow-ups as well on her blog. Responding to critics, she updated her blog with a statement that reads, in part:

I just wanted to explain a couple of things. First, my kids are in no way hurt for things.... They have reacted by making gifts for each other and packing them into each other's stockings stealthily(偷偷地). They are learning exactly what we wanted them to learn, because they are not moving around feeling sorry for themselves. They are thinking of others.

    The second thing I wanted to explain is why I wrote this post. Some people seem to think I wrote this for attention. Ummm, the attention you get from posts like this is not good and actually extremely difficult to deal with.... The reason I wrote this post is I want to empower parents to feel like it's okay to take a stand. ... I wanted to share what we are doing, so any parents that feel they are struggling with the same issues in their home can see what others are doing and get ideas for their family. My intention is to help support other parents and to raise amazing kids.

C

    There's a new frontier in 3D printing that's beginning to come into focus: food. Recent development has made possible machines that print, cook, and serve foods on a mass scale. And the industry isn't stopping there.

Food production

    With a 3D printer, a cook can print complicated chocolate sculptures and beautiful pieces for decoration on a wedding cake. Not everybody can do that — it takes years of experience, but a printer makes it easy. A restaurant in Spain uses a Foodini to "re-create forms and pieces" of food that are "exactly the same," freeing cooks to complete other tasks. In another restaurant, all of the dishes and desserts it serves are 3D-printed, rather than farm to table.

Sustainability(可持续性）

    The global population is expected to grow to 9.6 billion by 2050, and some analysts estimate that food production will need to be raised by 50 percent to maintain current levels. Sustainability is becoming a necessity. 3D food printing could probably contribute to the solution. Some experts believe printers could use hydrocolloids (水解胶体) from plentiful renewables like algae(藻类) and grass to replace the familiar ingredients(烹饪原料). 3D printing can reduce fuel use and emissions. Grocery stores of the future might stock "food" that lasts years on end, freeing up shelf space and reducing transportation and storage requirements.

Nutrition

    Future 3D food printers could make processed food healthier. Hod Lipson, a professor at Columbia University, said, "Food printing could allow consumers to print food with customized nutritional content, like vitamins. So instead of eating a piece of yesterday's bread from the supermarket, you'd eat something baked just for you on demand."

Challenges

    Despite recent advancements in 3D food printing, the industry has many challenges to overcome. Currently, most ingredients must be changed to a paste(糊状物) before a printer can use them, and the printing process is quite time-consuming, because ingredients interact with each other in very complex ways. On top of that, most of the 3D food printers now are restricted to dry ingredients, because meat and milk products may easily go bad. Some experts are skeptical about 3D food printers, believing they are better suited for fast food restaurants than homes and high-end restaurants.

    Youth football team members rescued more than two weeks after sudden flooding trapped them in a cave in Thailand are now being well looked after at a hospital in the northern city of Chiang Rai. In addition to treating the boys for potential body fluid loss, inadequate nutrition and lack of oxygen, their doctors also plan to closely monitor them for symptoms of diseases that may have been infected by animals living in the cave.

    "The next step is to make sure those kids and their families are safe, because living in a cave provides a different environment, which might contain animals that could transmit…disease," said the local hospital. The boys and their family members have been told to watch for symptoms such as headache, nausea(反胃), muscle pain or difficulty breathing, the reports added.

    Yet based on the location where the boys were trapped—more than four kilometers from the cave complex's main entrance, past some fully submerged passages—and the fact they have been swimming out wearing full scuba face masks, it seems unlikely that they were living with bats in the cave or breathed in bat-associated bacteria during their rescue, several infectious disease experts said. "It's hard to imagine bats got that deep into the cave because of all those narrow passageways, but it is possible," says Ian Lipkin, an animal expert and professor at the Mailman School of Public Health at Columbia University. "It's unlikely that there would be many animals in there," notes Jonathan Epstein, a doctor at EcoHealth Alliance, a nonprofit organization that studies diseases and how to prevent them. Bats typically like to rest in areas they can easily enter and exit, not in places that fully flood, he adds.

    Bats in Thailand have been linked with a wide range of viruses that are similar to severe acute respiratory syndrome (SARS)—Lipkin says. But it seems more likely the boys would have been exposed to infection-causing bacteria when they swam through the dirty water with cuts and scrapes. "If you are trying to prioritize issues with respect to health care for these kids, number one would be psychological damage and second will be bacterial infections from the cuts and scrapes they may have encountered." Lipkin says.

    The cost of living in countries like the U.S., Britain, Australia or France has skyrocketed (飞涨) in recent years. However, if you are willing to leave your country and explore life in other areas, there are many countries boasting an extremely cheap and low cost of living. Check out the following countries when planning your next move.

    Ecuador

    Ecuador is similar to the United States in many ways, and as a result has become a popular American getaway. They even use the U.S. dollar for their currency. In this country, it is cheaper to eat out every day than cook for yourself on a budget in the U.S. You can even buy a property for around $50,000 and hire someone to tend to it while you are away.

    Mexico

    Although the country is known for its poverty-ridden streets, there are many wealthier areas that provide every facility you might need. You can live off a mere $700 to $800 a month in Mexico, and this includes rent for a nice place, facilities, food, and any sightseeing.

    Argentina

    Argentina has been a popular tourist destination for many years. It has great restaurants, a rich history, and many places to explore. Not to mention, it is not very expensive to live there. A decent sized home can cost around $139,000, which is relatively cheap in relation to homes in the United States.

    Vietnam

    Vietnam has also become a popular tourist destination in recent years. There are many million dollar luxury homes by the beach, but you can find a modest flat or small home for around $25,000 to $50,000 if you search in the right areas.

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