Transparent animals let light pass through their bodies the same way light passes through a window. These animals typically live between 

the surface of the ocean and a depth of about 3,300 feet—as far as most light can reach. Most of them are extremely delicate and can be 

damaged by a simple touch. Sonke  Johnsen, a scientist in biology, says, “These animals live through their life alone. They never

touch anything unless they're eating it, or unless something is eating them.”

And they are as clear as glass. How does an animal become see-through? It's trickier than you might think.

      The objects around you are visible because they interact with light. Light typically travels in a straight line. But some materials slow and

 scatter(散射) light, bouncing it away from its original path. Others absorb light, stopping it dead in its tracks. Both scattering and absorption

 make an object look different from other objects around it, so you can see it easily.

But a transparent object doesn't absorb or scatter light, at least not very much, Light can pass through it without bending or stopping.

That means a transparent object doesn't look very different from the surrounding air or water. You don't see it —you see the things

behind it.

   To become transparent, an animal needs to keep its body from absorbing or scattering light. Living materials can stop light because they

contain pigments(色素) that absorb specific colors of light. But a transparent animal doesn't have pigments, so its tissues won't absorb

 light. According to  Johnsen, avoiding absorption is actually easy. The real challenge is preventing light from scattering.

Animals are built of many different materials—skin, fat, and more—and light moves through each at a different speed. Every time light

moves into a material with a new speed, it bends and scatters.Transparent animals use different tricks to fight scattering. Some animals are

simply very small or extremely flat. Without much tissue to scatter light, it is easier to be see--through. Others build a large, clear mass of

 non-living jelly-lie(果冻状的)material and spread themselves over it .

    Larger transparent animals have the biggest challenge, because they have to make all the different tissues in their bodies slow down light 

exactly as much as water does. They need to look uniform. But how they're doing it is still unknown. One thing is clear for these larger animals, staying transparent is an active process. When they die, they turn a non-transparent milky white.

    The most wonderful time of the year (the Christmas season) is also the most stressful for lots of people struggling to fit in increased end-of-the-year workloads, holiday parties, shopping, guest hosting, travel, and seeing friends and relatives who you'd otherwise avoid. Luckily, the same coping mechanisms that can help relieve stress and find better balance during the year also work for this holiday.

    Here are four tips to finding work—life balance during this season.

1） Know and avoid your stressors(压力源).

    If cooking for a large group, baking cookies, or Christmas shopping are your key stressors, don't do them. Lots of grocery stores will help cater your next holiday meal. Bakeries exist for a reason, and gift cards are much appreciated by all. Stop trying to live up to someone else's expectations of the holidays and stay merry by outsourcing the pain points.

2） Get flexible at work.

    If you normally travel an hour each way at work, see if your boss will let you work from home in this special season to save time, get more done, and reduce your stress. Or ask if you can start work from home in the mornings and come to the office later in the day to avoid peak hours. Leave early and finish your day from the comfort of home, too.

3） Make your own list and check it twice.

    Make a to-do list for yourself, for both work and life, then divide it up into categories like “must do,” “want to do,” and “feel obligated to do.” If you can remove any or all of your required list, the rest of your to-dos will start to look a lot easier.

    Once you have your lists in order, it's time to start crossing things off. In between your must-dos tasks, include a want-to-do task to break things up and re-energize yourself. Yes, this holiday is a time to give back to others and be selfless, but there's still a bit of “self” in selflessness, isn't there?

4）If you start to feel a cold coming on, come to a full and complete stop.

    The best way to stop a cold from exploding into a weeks-long sickness is to stop it at the start. Unless a task or activity is an absolute necessity, cancel your plans, put away your to-do lists, and settle in for a long winter's nap. A day of rest and enough sleep is the only thing you NEED right now. Feel bad about declining holiday invitations from your friends and family? You're doing them a favor. Once people hear that you're sick, they'll be glad you stayed away — they don't want to come down with a cold during the holiday any more than you do!

    It is certain that you can feel less stressed in this hectic season when you stop expecting so much from yourself, because nobody puts so much expectation on you as you do to yourself. Trying to satisfy clients, co-workers, friends and family can be draining and can finally negatively impact you in many ways. When you simplify things up front, you relieve the stress off of yourself and everyone else in the process. Stop focusing on what you feel like you're obligated to do, and start focusing on what you want and need to do, the load will get lighter and you will feel a lot merrier under the Christmas tree.

    From the modern London Eye to the historic Tower of London, below are London's most visited tourist attractions. Many of London's top attractions are free, making them affordable places to learn about some culture. Whether you prefer history or modern art, you'll find it at one of these must-visit spots.

British Museum

    The world-famous British Museum exhibits the works of man from prehistoric to modern times, from around the world. The most interesting and exciting parts include the Rosetta stone, the Parthenon sculptures and the mummies in the Ancient Egypt collection. Some exhibitions require tickets.

National Gallery

    The greatest and most beautiful building of Trafalgar Square, London's National Gallery is a vast space filled with Western European paintings from the 13th to the 19th centuries. In this impressive art gallery you can find works by masters such as Van Gogh, da Vinci, Botticelli, Constable, Renoir, Titian and Stubbs. Some exhibitions require tickets.

Tate Modern

    Sitting on the banks of the Thames is the Tate Modern, Britain's national museum of modern and contemporary art. Its unique shape is due to it being a power station before. The gallery's restaurants offer wonderful views across the city. Some exhibitions require tickets.

Coca-Cola London Eye

    The Coca-Cola London Eye is a major feature of London's skyline. It has some of London's best views from its 32 capsules, each weighing 10 tonnes and holding up to 25 people. Climb aboard for a breathtaking experience, with an unforgettable view of more than 55 of London's most famous landmarks -- all in just 30 minutes!

Madame Tussauds

    At Madame Tussauds, you'll come face-to-face with some of the world's most famous faces. From Shakespeare to Lady Gaga you'll meet influential figures from show business, sport, politics and even royalty. Strike a pose with Usain Bolt, get close to One Direction or receive a once-in-a-lifetime audience with Her Majesty the Queen.

    He's an old cobbler(修鞋匠) with a shop in the Marais, a historic area in Paris. When I took him my shoes, he at first told me: “I haven't time. Take them to the other fellow on the main street; he'll fix them for you right away.”

    But I'd had my eye on his shop for a long time. Just looking at his bench loaded with tools and pieces of leather, I knew he was a skilled craftsman(手艺人). “No,” I replied, “the other fellow can't do it well.”

    “The other fellow” was one of those shopkeepers who fix shoes and make keys “while-U-wait” — without knowing much about mending shoes or making keys. They work carelessly, and when they have finished sewing back a sandal strap(鞋带), you might as well just throw away the pair.

    My man saw I wouldn't give in, and he smiled. He looked at my shoes, had me write my name on one shoe with a piece of chalk and said, “Come back in a week.”

    I was about to leave when he took a pair of soft leather boots off a shelf.

    “See what I can do?” he said with pride. “Only three of us in Paris can do this kind of work.”

    When I got back out into the street, the world seemed brand-new to me. He was something out of an ancient legend, this old craftsman with his way of speaking familiarly, his very strange, dusty felt hat, his funny accent from who-knows-where and, above all, his pride in his craft.

    These are times when nothing is important but the bottom line, when you can do things any old way as long as it “pays”, when, in short, people look on work as a path to ever-increasing consumption(消费) rather than a way to realize their own abilities. In such a period it is a rare comfort to find a cobbler who gets his greatest satisfaction from pride in a job well done.

    I have never been a fan of the phrase, "No pains, no gains." I prefer the alternative version, "No pains, no pains!" When it comes to exercise, for example, I learned years ago that pushing through pain was more likely to lay me up with an aching back than to leave me feeling strong and healthy.

    But there are times when stressful situations actually do lead to greater happiness. A new study published in the Journal of Happiness Studies found that people who pursue (追求) goals that are tough to achieve feel more stress in the moment, but greater happiness in the aftermath particularly if they feel connected to others along the way.

    The researchers concluded that happiness increases when people develop greater competence in something and greater competence only comes when people keep on working through the stress-inducing phase of trying, fighting, and trying again to learn and grow. If the psychological needs to be autonomous or self-directed, and to be connected to others are met, the momentary stresses will be less acute, and the resulting happiness will be more lasting.

    This idea reminds me of another often quoted phrase, which I have heard in reference to challenging tasks, such as exercise —"Fill-in-the-blank-stressful-task is the worst thing to do, but the greatest thing to have done." In other words, suffering from the stress of an exercise class that leaves us feeling sweat-drenched shouldn't lead us to walk away from exercising. And if we can pass those stressful moments with a friend, colleague or family member who is trustworthy and supportive, all will be better.

    As I travel my positive path, I'm certainly not seeking out stress. But life will offer me plenty of it, whether I ask for it or not. My task is to choose wisely when and how to face it head-on, knowing that happiness awaits on the other side.

    The following ads come from UW(University of Washington)newspaper called The Daily.