Nowadays people are putting our water supply in danger, because they use too much water or throw rubbish into the river. You may wonder how paving(铺砌) a road can lead to less usable fresh water. A major part of thewaterweuse every day is groundwater. Groundwater does not come from lakes or rivers. It comes from underground. The more roads and parking lots we pave，the less water can flow into the ground to become groundwater.

    Human activity is not responsible for all water shortages(短缺). Drier climates are of course more likely to have droughts(干旱) than areas with morerainfall，but in any case, good management can help to make sure there is enough water to meet our basic needs.

    Thinking about the way we use water every day can make a big difference，too. In the United States，a family of four can use 1.5 tons of water a day! This shows how much we depend on water to live，but there's a lot we can do to lower the number.

    You can take steps to save water in your home. To start with，use the same glass for your drinking water all day. Wash it only once a day. Run your dishwasher (洗碗机)only when it is full. Help your parents fix any leaks in your home. You can even help to keep our water supply clean by recycling batteries instead of throwing them away.

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    Can exercise during childhood protect you against memory loss many decades later?Exercise early in life seems to have lifelong benefits for the brain,in rats at least.

     “This is an animal study,but it shows that physical activity at a young age is very important—not just for physical development,but for the whole lifelong track of cognitive（认知的）development during ageing,”says Martin Wojtowicz of the University of Toronto,Canada.“In humans,it may delay the appearance of Alzheimer's symptoms（阿茨海默氏症）,possibly to the point of preventing them.”

    Wojtowicz's team divided 80 young male rats into two equal groups,and placed running wheels in the cages of one group for a period of six weeks.Around four months later—when the rats had reached middle age—the team taught all the rats to connect an electric shock with being in a specific.When placed in the box,they froze with fear.

    Two weeks later,the team tested the rats in three situations: exactly the same box in the same room,the same box with the room arranged differently,and a completely different box in a different room.

    The rats without access to a running wheel when they were young now froze the same percentage of times in each of these situations,suggesting they couldn't remember which one was dangerous.But those that had been able to run in their youth froze 40 to 50 percent less in both changed box settings.

    "The results suggest the amount of physical activity when we're young,at least for rats,has influence on brain and cognitive health—in the form of better memories—when we're older,"says Arthur Kramer of Northeastern University in Boston,who has found that,in humans,exercise promotes the growth of new brain cells.

    A parent might place his daughter's drawing on the fridge out of a love for his child rather than for the wonderful image, but for many people, that children art is actually quite amazing. In fact, adult artists were often inspired by children's drawing. For the museum-goers out there who tend to point to a piece of modern art and say, “My kid could have made that !”It's worth remembering that often, that's actually just what the artist had in mind.

    For many kids, drawing is exciting not because of the final product it leads to, but because they can live completely in the world of their drawing for a few minutes. Even children are scribbling(涂鸦), they're representing through action, not through pictures. Liane Alves, a prekindergarten teacher, recalled a student who presented her with a drawing featuring a single straight line across the page. Alves assumed the child hadn't given too much thought to the drawing until he explained that the line was one of the stems (茎) from The Princess and the Pea, one of the fairy tales they read in class.

    Maureen Ingram, who's a preschool teacher at the same school,said her students often tell different stories about a given piece of art depending on the day, perhaps because they weren't sure what they intended to draw when they started the picture. “We as adults will often say, 'I'm going to draw a horse, 'and we set out. . . and get frustrated when we can't do it, ”Ingram said. “Children seem to take a different approach, where they just draw, and then they realize, 'it is a horse. '”

    And what about those odd or scary-looking drawings? Does that mean kids are telling themselves stories that are odd or scary? It's hard to say, but it's rarely a good idea to over-interpret it. Ellen Winner, a psychology professor, pointed to parents who worry when their kid draws a child bigger than the adults. What's most important to remember is that“children's art has its own logic,” Winner said. “Children are not being crazy. ”

    Mexico sites on an island plateau (高原) surrounded by volcanic peaks, which makes air quality a constant concern for people who live there.

    In April, the country took a decisive step toward improving air quality by enacting a temporary ban on private and Federal vehicles in the city. The rule forbids people from driving in the city one day each week and one Saturday each month. Electric vehicles, government service vehicles, public transport options and school buses are not included in the ban.

    Mexico City isn't the first urban center to be involved in car-free living. But the ban is more than an awareness-raiser. It was enacted with the direct aim of solving air pollution. In March, the city sank into a deep brown haze of smog when the pollution levels passed the 200 mark. The city ordered some 1.1m of the area's 4.7m cars off the streets and also offered free bus and subway rides.

    Mexico City's temporary vehicle ban raises questions about the best ways to improve urban air quality. India, whose citizens breathe some of the world's dirtiest air, has tried a variety of solutions. Earlier this year, the Indian government started a 4% sales tax on new-car purchases.

    Beijing also has experience with cleaning the air. The city introduced alternate-day rules in advance of the 2008 Olympic Games, which produced good results. And then there is the London congestion (拥挤) charge, introduced in 2003. The charge has had a measurable effect on air quality.

For now, Mexico City — named by the United Nations as the most polluted city on the planet in 1992 — can look forward to the start of the region's rainy season, when daily showers will help lo clean the air.

    What is language for? Some people seem to think it's for practicing grammar rules and learning lists of words—the longer the lists, the better. That's wrong. Language is for the exchange of ideas and information. It's meaningless knowing all about a language if you can't use it freely. Many students I have met know hundreds of grammar rules, but they can't speak correctly or fluently（流利地）. They are afraid of making mistakes. One shouldn't be afraid of making mistakes when speaking a foreign language. Native speakers make mistakes and break rules, too. Bernard Shaw once wrote,  “Foreigners often speak English too correctly. ” But the mistakes that native speakers make are different from those that Chinese students make. They're English mistakes in the English language. And if enough native speakers break a rule, it is no longer a rule. What used to be wrong becomes right. People not only make history, they make language. But a people can only make its own language. It can't make another people's language. So Chinese students of English should pay attention to grammar, but they shouldn't overdo it. They should put communication first.

    Good evening, everyone. As we celebrate International Dog Day, I want to focus on a specific type of dog that is perhaps the most misunderstood breed in the world—the pit bull. It makes me unhappy that public opinion about pit bulls is that they are extremely dangerous. If a pit bull is that meaning, that's only as a result of being taught to fight other dogs to the death in dogfights. The truth is that a pit bull is very faithful and loving.

    A pit bull is a cross between a terrier and a bulldog. That's why their coloring can be different from one dog to the next. Pit bulls all have short fur. They came over to the U.S. from Britain with their immigrant farming families. Pit bulls guarded the cows, sheep and family from thieves. They were beloved members of the family because they were so faithful and loving. They were also very strong, especially their jaws.

    Unfortunately, bad men turned a good thing into a bad thing. Since these dogs would do anything to please their masters, it was easy for these men to teach the dogs to be killers. Dogs fought other dogs in fenced "pits". Dog fighting became illegal in the U.S. in 1875, but it still goes on. Today, pit bulls continue to be farm dogs, but they are also trained as police dogs, search dogs and of course they make great pets!

    I have a pit bull myself. Piper is the friendliest dog you'll ever meet. He sleeps next to me at night. Even though he is an adult dog now, he still acts like a puppy sometimes. He would never bite me or anyone else! With proper and loving training, pit bulls are the best pets ever!