Most people know precious gemstones (宝石) by their appearances. An emerald flashes deep green, a ruby seems to hold a red fire inside, and a diamond shines like a star. It's more difficult to tell where the gem was mined, since a diamond from Australia or Arkansas may appear the same to one from the Democratic Republic of the Congo. However, recently, a team of scientists has found a way to identify a gemstone's origin.

    Beneath the surface of a gemstone, on the tiny level of atoms and molecules（分子）, lie clues (线索) to its origin. At this year's meeting of the Geological Society of America in Minneapolis, Catherine McManus reported on a technique that uses lasers (激光) to clarify these clues and identify a stone's homeland. McManus directs scientific research at Materialytics, in Killeen, Texas. The company is developing the technique. “With enough data, we could identify which country, which mining place, even the individual mine a gemstone comes from,” McManus told Science News.

    Some gemstones, including many diamonds, come from war-torn countries. Sales of those “blood minerals” may encourage violent civil wars where innocent people are injured or killed. In an effort to reduce the trade in blood minerals, the U.S. government passed law in July 2010 that requires companies that sell gemstones to determine the origins of their stones.

    To figure out where gemstones come from, McManus and her team focus a powerful laser on a small sample of the gemstone. The technique is called laser-induced breakdown spectroscopy. Just as heat can turn ice into water or water into steam, energy from the laser changes the state of matter of the stone. The laser changes a miniscule part of the gemstone into plasma, a gas state of matter in which tiny particles（微粒）called electrons separate from atoms.

    The plasma, which is superhot, produces a light pattern. (The science of analyzing this kind of light pattern is called spectroscopy.) Different elements produce different patterns, but McManus and her team say that gemstones from the same area produce similar patterns. Materialytics has already collected patterns from thousands of gemstones, including more than 200 from diamonds. They can compare the light pattern from an unknown gemstone to patterns they do know and look for a match. The light pattern acts like a signature, telling the researchers the origin of the gemstone.

    In a small test, the laser technique correctly identified the origins of 95 out of every 100 diamonds. For gemstones like emeralds and rubies, the technique proved successful for 98 out of every 100 stones. The scientists need to collect and analyze more samples, including those from war-torn countries, before the tool is ready for commercial use.

    Scientists like Barbara Dutrow, a mineralogist from Louisiana State University in Baton Rouge, find the technique exciting. “This is a basic new tool that could provide a better fingerprint of a material from a particular locality,” she told Science News.

    Taking a math test can be pretty stressful. Even if you know the material, you can still get the problem wrong. {#blank#}1{#/blank#} Therefore you'll improve your grades, as well as your math skills.

    Write it out

    You can also check a math problem by writing everything out on paper. {#blank#}2{#/blank#} Writing out math problems reduces your chances of missing anything to the lowest possible level, which is a common cause of incorrect answers.

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    Make sure your answers work by doing the opposite procedure of what your problem calls for, including the answer you got the first time around. In other words, you would use the opposite of this addition problem—subtraction (减法)—to determine whether your answer is the correct one.

    Plug in（代入）

    You may find that a variable isn't good enough or have a problem where you have to solve for a variable (变量). {#blank#}4{#/blank#} This is the only real way to assure yourself that the answer you've found is correct.

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    If the result of a problem seems to make no sense, it indicates that the answer is incorrect. For example, if you get an answer in the millions and you know it should be in the thousands, you've likely misplaced a point. Go back through the work on your paper to make sure all of your formulas and calculations are correct. If everything looks okay, do the problem again and compare the result of the second try to what you've got on the first try.

A. Do the opposite.

B. Correct the answers.

C. Plug the variable in the equation (方程) to check it out.

D. Knowing how to go through your math test and checking your work can save you from handing in a test full of mistakes.

E. It also helps you to figure out everything after you have already finished the test.

F. Check for a reasonable answer

G. This helps you to know what information you have and what information you need to solve.

    Group exercise is one of the most effective ways to improve physical fitness and sustain a healthy lifestyle.Group exercise is challenging, yet fun and empowering! Of course everyone knows that exercise is good for the body.However, studies have shown that when exercise is performed in groups, it's not only great for improving physical health but for psychological health. It's an opportunity to be social, release endorphins  , and improve your strength. Additionally, group exercise creates a community feel and the shared common goal motivates participants to work hard. The instrumental support of taking on a fitness journey with others proves more effective than going to the gym alone.

    Another beneficial aspect of group exercise is the informational support participants receive from the instructor.Many people fear the gym because they feel lost and don't want to embarrass themselves. If you feel you can relate, then group training is an even better option for you. It's a great opportunity to learn more about fitness through the clear instruction and supervision of a fitness instructor. If you're tired of wandering around the gym wasting time and becoming bored, !you can attend an upbeat group fitness class that'll keep your workout on track. Don't let fitness frighten you!

    If you're serious about wanting to live a healthy lifestyle, it's extremely important to surround yourself with people who'll provide you with the proper emotional support. I wouldn't scold anyone for deciding to party on weekends and in turn I wouldn't expect anyone to give offence to me for focusing on my health. Surround yourself with people who uplift, encourage and understand you! Make fitness even more fun by trying something new or any group fitness class, with a friend. Plan to go for a jog together. Then try a fun healthy restaurant or fresh juice bar! Fitness can be both fun and social!

    Surrounding yourself with people who'll provide you with respect support can be very beneficial while working towards reaching health and fitness goals. First, decide to do it for yourself and work towards staying positive. Then make sure the people you surround yourself with are supportive. Don't let negativity ruin your motivation.

    Teenagers around the world can be happy with the news that the brain will ignore parents' order when they tap on their smartphones. A new scientific study from the University College London has shown that humans may temporarily go deaf when they're focusing on something visual at the same time.

    The researchers played the normal-volume sounds in the background. And 13 volunteers experienced inattentional deafness as their visual tasks became increasingly difficult. “We found that when volunteers were performing the demanding visual task, they were unable to hear sounds that they would normally hear,” Maria Chait said in a statement. “The brain scans showed that people didn't filter out the sounds on purpose. They were not actually hearing them in the first place.”

    The findings, published in the Journal of Neuroscience, suggest that the centre of sights and the center of sounds share limited resources. Inattentional deafness is a common everyday experience and the study explains why, according to UCL Institute of Cognitive Neuroscience Professor Nili Lavie.

    If you try to talk to someone focusing on a book, game, or television program and don't receive a response, they aren't necessarily ignoring you. They simply might not hear you at all. This could also explain why you might not hear your bus or train stop being announced if you're absorbed in your phone, book or newspaper. However, some loud sounds will still be able to break through.

    Some situations could become potentially dangerous when the quieter ones go unheard. As you can imagine, in the operating room, when a doctor concentrates on his work, he might not hear the equipment beeping. It also applies to drivers who concentrate on complex directions. Fortunately, experts have given us some useful tips on preventing such situations.

    US Space Agency Returns to Mars with InSight Lander

    The American space agency's lander InSight is to arrive on Mars at the end of November, 2018. Unlike NASA's Curiosity rover, InSight will not move across the planet's surface. Instead it will become the first spacecraft (航天器) used only for exploring underground. It will study the geography of Mars, searching for signs of Martian (火星的) quakes.

    InSight is a project of U.S. and Europe. It is 6 meters long and 1.5 meters wide. It weighs 360kilograms. InSight's 1 .8 -meter robotic arm will put two experiments in place. Both are designed toexplore underground of Mars to learn about the inside.

    Bruce Banerdt is the lead scientist for InSight. He viewed the spacecraft as a robot that can take care of itself. "It's got its own brain. It's got an arm. It can listen and feel things. It pulls its own power out of the sun," he noted.

    Thomas Zurbuchen, one of the heads in NASA, said the results of the InSight project could "change the way we think about the inside of our earth." But, first, InSight will have to land on Mars undamaged. Since Mars exploration started in the 1960s, only about 40 percent of the explorations have succeeded.

    Tom Hoffman is InSight's project manager. He said earlier successes do not lessen the concerns for each new exploration. "That we ve done it before doesn't mean we're not nervous and excited about doing it again," he said.

    InSight will enter the Martian atmosphere (大气层) traveling at 19,800 kilometers an hour. It will slow for landing on a wide flat area on Mars. If all goes well, InSight could carry out its experiments in about 10 weeks. The InSight project is expected to continue for one Martian year, about two years on Earth.

    Big Ben is not the name of a man. It is the name of a huge(庞大的)clock in London. London is the capital of England. This clock has four faces. So，no matter where you stand, you can read the time on the face of Big Ben. Each face is the size of a double decker(层)bus. The hands are about four metres long. It is about the size of two people standing on top of each other. If you go to London, you may want to visit the Houses of Parliament(国会大厦). There you will find Big Ben sits at the top of the clock tower(塔)in the Houses of Parliament. Maybe you will hear it as well as see it. The huge clock makes such a loud noise. "Ding dong, ding dong, "it goes every quarter of an hour.

    The clock was named after a big man. He was Sir Benjamin Hall. This man did much building work in London many years ago.