27 world records were broken during the 2016 Summer Olympics. They fell in swimming running weightlifting and a number of other sports. But how do athletes continue to get better, jump longer, run or swim faster? Although scientists say that athletes have reached their limits (极限), records continue to fall. In almost every sport, athletes have become better since the Olympic Games begin.

    One reason is that and more people than ever before have access to sports and exercise. They start at an earlier age and can compete longer in their sport. In schools more and more natural talents can be discovered.

    Besides, athletes can train full-time instead of training after works. As medicine improves, athletes can stay in competition for a longer time and overcome injuries (损伤) faster. Many top athletes achieve their best results later on in their careers. Technology has also helped improve scores.

    One of the most important reasons, however, is the human mind. We always want to be better and faster that someone else. It can release (释放) great power in our body.

    So, where are limits? One of the most difficult questions to answer is how fast a human being can run. Many years ago, nobody thought a person could ever run within 10 seconds in the 100-metre race. But the record was broken at the 1968 Mexico City Olympics. A t the moment Sustain Bolt is the fastest man on Earth at 9.58 seconds. Scientists say that 9.48 may be the limit, but, who knows, maybe some runners will one day run within 9 seconds.

    In today's world, almost everyone knows that air pollution and water pollution are harmful to people. However, not all the persons know that noise is also a kind of pollution, and that is harmful to human, too.

    People who work and live under noisy conditions usually become deaf(聋的). Today, however, scientists believe that 10 percent (百分比) of workers in Britain are being deafened by the noise where they work. Many of the workers who print newspapers and books, and who weave(织) cloth become deaf. Quite a few people living near airports also become deaf. Recently(近来) it was discovered that many teenagers in America could hear no better than 65-year-old people, for these young people like to listen to "pop" music and most of "pop" music is a kind of noise. Besides(除此之外), noise produced by jet planes or machines will make people's life difficult and unpleasant, or even make people ill or even drive them mad.

    Nowadays the government in many countries have made laws to reduce or control(控制) noise and make it less than 85 decibels(分贝).  In China, the people's government is trying to solve not only air and water pollution problems but also noise pollution problem.

    Smartphone apps certainly make our lives easily. However, sometimes they can also cause problems.

    Recently, the Ministry of Education(教育部) announced that teacher should not use Wechat or QQ to give homework or ask parents to grade kids's homework.

    Many teachers in China depend heavily on apps to give homework. The kids have to do their homework, stare at a phone screen. For example, Liu Yanming, a sixth-grade student in Shanghai, usually has to use his mother mobile phone to do his homework. Liu Yong, his father is afraid that apps could distract kids by encourage them to watch something unrelated(无关) to homework. "He is just 12 years old, and I do not want to buy him a cellphone," Liu told China Daily. "I also worry that my son's eyesight will be hurt because of staring at the phone for too long."

    However, Bai Yueping, president of Shanghai Tangzhen Middle School, defended (辩护) using apps for homework. Parents can understand their kids' learning situation and supervise (监督) them to finish the homework, Bai told China Daily.

    Zhang Siqi is a student from Renmin University of China.       "The use of WeChat or QQ to give homework is convenient," Zhang told China Daily. "We're living in an Internet era （时代）. It's unwise to refuse to take advantage of technology."

    Chu Zhaohui, a researcher at the National Institute of Education Sciences, believed that it is impossible to stop the use of electronic devices(设备) in the Internet era. However, students should use them when it's necessary, such as when they do listening exercises. "We don't want to stop outline education models, but they must be used carefully. Modern technologies are good... but they should not steal the show(喧宾夺主) ," he told China Daily.

    How do animals communicate (交流) with each other? They have special "languages". In fact, they are using signals (信号). Different signals have different meanings.

    For example, when a bee (蜜蜂) finds food somewhere, it flies home quickly. It can't speak, but it can tell others by dancing. Other bees will know where and how far the food is.

    Some animals show their feelings by making sounds. For example, when a dog is angry, it barks (吠). At this time, you'd better stay away from it. Birds can make several sounds to show different meanings. Sometimes we humans show feelings by making sounds instead of by speaking. For example, when we feel painful (疼痛的), we cry "Oh!" or“Ouch!".

    We humans have languages and words. We can communicate face to face by speaking. We can write down what happened in order to remember clearly. We can also send (寄) messages to people far away. But animals can't.