However wealthy we may be, we can never find enough hours in the day to do everything we want. Economics deals with this problem through the concept of opportunity cost, which simply refers to whether someone's time or money could be better spent on something else.

    Every hour of our time has a value. For every hour we work at one job we could quite easily be doing another, or be sleeping or watching a film. Each of these options has a different opportunity cost—namely, what they cost us in missed opportunities.

    Say you intend to watch a football match but the tickets are expensive and it will take you a couple of hours to get to and from the stadium. Why not, you might reason, watch the game from home and use the leftover money and time to have dinner with friends? This—the alternative use of your cash and time—is the opportunity cost.

    For economists, every decision is made by knowledge of what one must forgo—in terms of money and enjoyment—in order to take it up. By knowing precisely what you are receiving and what you are missing out on, you ought to be able to make better-informed, more reasonable decisions. Consider that most famous economic rule of all: there's no such thing as a free lunch. Even if someone offers to take you out to lunch for free, the time you will spend in the restaurant still costs you something in terms of forgone opportunities.

    Some people find the idea of opportunity cost extremely discouraging: imagine spending your entire life calculating whether your time would be better spent elsewhere doing something more profitable or enjoyable. Yet, in a sense it's human nature to do precisely that—we assess the advantages and disadvantages of decisions all the time.

    In the business world, a popular phrase is “value for money.” People want their cash to go as far as possible. However, another is fast obtaining an advantage: “value for time.” The biggest restriction on our resources is the number of hours we can devote to something, so we look to maximize the return we get on our investment of time. By reading this passage you are giving over a bit of your time which could be spent doing other activities, such as sleeping and eating. In return, however, this passage will help you to think like an economist, closely considering the opportunity cost of each of your decisions.

    The past two decades have seen astronomers' catalogue of planets expand over two hundred times, as new techniques and better telescopes have found more than 2,000 of them orbiting stars other than the sun. But in the solar system itself, the list of planets has actually shrunk, Pluto（冥王星）having been downgraded from that status in 2006. The number of the sun's planetary companions has thus fallen from nine to eight.

    Now, a pair of astronomers from the California Institute of Technology think they have evidence that will restore the sun's record to its previous value. Their analysis of objects orbiting in the Kuiper Belt（柯伊伯带）, a ring of frozen asteroids（小行星）that circle beyond the orbit of Neptune (and of which Pluto is now regarded as the largest member), suggests to them that something about ten times as massive as Earth has changed those orbits. If you knew where to look, this planet-sized object would be visible through a suitable telescope. And Konstanin Batygin and Michael Brown believe they do know.

As they write in the Astronomical journal, they have analyzed the orbits of Kuiper Belt objects and found six that behave in a peculiar way. As the diagram shows, the points of closest approach of these objects to the sun, known as their perihelia（近日点）, almost coincide. Moreover, these perihelia all lie near the ecliptic（黄道）—the plane of Earth's orbit and also, approximately, that of the other planets—while the objects' orbits are all angled at 30° below the ecliptic. The chance of all this being a coincidence, the two researchers estimate, is about seven in 100,000. If it is not a coincidence, it suggests the six objects have been guided into their orbits by the gravitational intervention of something much larger.

    A computer analysis Dr Batygin and Dr Brown performed suggests this something is a planet weighing 5-15 times as much as Earth, whose perihelion is on the opposite side of the sun from the cluster, and which thus orbits mainly on the other side of the solar system from the objects its orbit has affected. This planet's perihelion would be 200 times farther from the sun than Earth's, and the far end of its orbit might be as much as six times that distance away. This gives a search zone, and Dr Batygin and Dr Brown are using Subaru, a Japanese telescope, to perform that search.

    Given other demands on Subaru's time, it might take five years for this search to find (or not find) the hypothetical planet. But looking at some existing data from. The Widefield Infrared Survey Explore, a satellite, might also show it, if it is there to be seen.

    Ironically, it was Dr Brown as much as anyone who was responsible for Pluto's downgrading, for he discovered Eris, an object almost as big as Pluto, in 2005.

    That discovery did much to damage Pluto's planetary proof. By his own admission, he was skeptical that the anomalies he and Dr Batygin have investigated actually would point to the existence of a replacement ninth planet. He is a skeptic no longer. Whether he is actually right may soon become apparent.

Plastic-Eating Worms

    Humans produce more than 300 million tons of plastic every year. Almost half of that winds up in landfills(垃圾填埋场), and up to 12 million tons pollute the oceans. So far there is no effective way to get rid of it, but a new study suggests an answer may lie in the stomachs of some hungry worms.

    Researchers in Spain and England recently found that the worms of the greater wax moth can break down polyethylene, which accounts for 40% of plastics. The team left 100 wax worms on a commercial polyethylene shopping bag for 12 hours, and the worms consumed and broke down about 92 milligrams, or almost 3% of it. To confirm that the worms' chewing alone was not responsible for the polyethylene breakdown, the researchers made some worms into paste(糊状物) and applied it to plastic films. 14 hours later the films had lost 13% of their mass — apparently broken down by enzymes (酶) from the worms' stomachs. Their findings were published in Current Biology in 2017.

    Federica Bertocchini, co-author of the study, says the worms' ability to break down their everyday food — beeswax — also allows them to break down plastic. “Wax is a complex mixture, but the basic bond in polyethylene, the carbon-carbon bond, is there as well,” she explains, “The wax worm evolved a method or system to break this bond. ”

    Jennifer DeBruyn, a microbiologist at the University of Tennessee, who was not involved in the study, says it is not surprising that such worms can break down polyethylene. But compared with previous studies, she finds the speed of breaking down in this one exciting. The next step, DeBruyn says, will be to identify the cause of the breakdown. Is it an enzyme produced by the worm itself or by its gut microbes(肠道微生物)?

    Bertocchini agrees and hopes her team's findings might one day help employ the enzyme to break down plastics in landfills. But she expects using the chemical in some kind of industrial process — not simply “millions of worms thrown on top of the plastic.”

    An unconventional new initiative(首创)in Canada will soon allow doctors to prescribe art to their patients—by giving them free access to a local museum. Wandering through the Montreal Museum of Fine Arts(MMFA), these patients and their loved ones will be able to feast their eyes on the peaceful collections of art.

    The initiative is the first of its kind in the world. And while you certainly can't replace a conventional treatment with a couple of paintings, the idea is for such “prescriptions” to assist a person's current treatment plan. Under the new program, members of the Francophone Association of Doctors in Canada(MFDC)will be able to hand out up to 50 prescriptions for their patients.

    These prescriptions will be available for those with a wide range of mental and physical illnesses, and each of them will allow two adults and two children to visit the museum for free.

    “By offering free admission to a safe, welcoming place, a relaxing experience, a moment of relief, and an opportunity to strengthen ties with loved ones, MMFA—MFDC Museum Prescriptions contribute to the patient's well-being and recovery, ”explains a news conference from the MMFA.

    It may look a lot like a marketing effort for the museum but there's also increasing evidence that the display of visual art, especially if it's showing nature, can have positive effects on health outcomes.

    In some ways, the benefits of looking at art appear a little similar to physical activity. A systematic review of clinical art therapy(疗法) found that visual art has significant and positive effects on depression, anxiety, mood, and self-esteem. Findings like these are slowly gaining attraction in the medical community, making artwork a higher priority(优先)in hospitals around the world.

    In the US, nearly half of all health care institutions have reported including art in health care programming, such as art therapy and the placement of visual art in hospitals. With spaces dedicated(专用的)to art therapy and also a medical consultation room, the MMFA already provides services for people with mental health issues, eating disorders, and Alzheimer's disease, just to name a few.

    I believe children have a very pure sense of humor. Well, I would like to share my experiences with you.

    The other day when I was out with a friend and her daughter Jane at a restaurant, Jane ordered an apple dessert, which was the most attractive dish in that restaurant. However, when she had a bite of the pie, she shouted loudly, "Mom, why does it look good but tastes like rubber?" My friend was uneasy. Then the waiter gave us another dish.

    A child can say funny things. I was at a friend's home for tea and her son, Mark, wanted her to get up. His mom replied, "I can't, because my foot has fallen asleep now." Little Mark then sang a song and asked, "Mom, will it wake up now?" We laughed together.

    The other day we were having an interesting conversation over lunch and my friend, an animal activist (积极分子) , was arguing with another group member. Her son Tommy suddenly stood up, “Mom, people should never be allowed to shoot extinct animals!” That was funny and the child did manage to turn the argument into a pleasant conversation.

    Well, children sometimes really surprise us. I think we can have a lot to learn from them. They can help us understand the simple world through their eyes.

Many people who are learning English want to know how to improve their reading skills. {#blank#}1{#/blank#} The effective way to improve your reading skills is to love reading and practice more.

Reading is one of the most important language skills because we use it to expand our vocabulary and improve the way we speak, write and listen. {#blank#}2{#/blank#} Seeing new ways to say things can help your understanding of English and improve not only your reading, but also your other language skills.

{#blank#}3{#/blank#} If we do not understand something, we can go over it again and even look up the words we do not know the meanings of. This means that we have a much greater chance of understanding information that we read than information that we hear.

So how do we practice reading so that our skill level improves? When we read a passage, we need to understand it. We must understand what the important words mean in each sentence. This does not mean that we need to know the meaning of every word. In many cases, we can find out what words mean from the words around them. {#blank#}4{#/blank#} To do this, we must understand how words work together. In many cases, we must "see" part of the message that has not been written, or we must form an opinion ourselves.

Reading is an essential part of your study of the English language. It can help you understand and learn more. It can also become a satisfying hobby.{#blank#}5{#/blank#}

A．Unfortunately, there is no easy way to do this.

B．Unlike listening, reading allows us to take our time.

C．Among the four skills, listening is the most important one.

D．It is hoped that you will love reading and improve your reading skills.

E．When we read, we can see how things are said, from sentence structure to expression.

F．If we do not, we will not be able to understand what the entire article or story is about.

G．Once we understand what the words mean, we need to figure out what the message is.