The word“day” has two meanings. When we talk about the number of days in a year,  we are using “day” to mean 24 hours. But when we talk about day and night,  we are using“day” to mean the time between sunrise and sunset. Since the earth looks like a ball,  the sun can shine on only half of it at a time. Always one half of the earth is having day and the other half night. A place is moved from day into night and from night into day over and over by the spinning（旋转）of the earth. At the equator（赤道）day and night are sometimes the same length（长度）. They are each twelve hours long. The sun rises at 6 o'clock in the morning and sets at 6 o'clock in the evening. For six months the North Pole is tilted（倾斜）toward the sun. In those months the Northern Hemisphere（半球）gets more hours of sunlight than the Southern Hemisphere. Days are longer than nights. South of the equator nights are longer than days. For the other six months the North Pole is tilted away from the sun. Then the Southern Hemisphere gets more sunlight. Days are longer than nights. North of the equator nights are longer than days. Winter is the season of long nights. Summer is the season of long days.

    It is easier to float in the ocean than in a swimming pool. Do you know why?

    Ocean water is full of salt - and salt water can hold up more weight than fresh water. See for yourself! (Ask an adult to help you.)  You will need: 3 cups warm water; A wide-mouth jar（罐）; Salt; A spoon; A potato. You will do: 1. Pour 1.5 cups of warm water into the jar. 2. Add 1/3 cup of salt.  3. Stir(搅拌) until the salt is completely dissolved(完全溶解) 4. Add another l.5 cups of water. Pour it slowly over the back of the spoon into the jar so that the two kinds of water will not mix together. 5. Softly place the potato into the jar. Don't drop it. What will happen: The potato will sink - but only halfway! Why: Salt water is heavier than fresh water, so it will stay on the bottom. The fresh water will remain on the top. The potato is heavier than the fresh water, but it is lighter than the salt water. That is why the potato will sink only halfway down.

    Several Asian hospitals are doing experiments with tele-medicine. Tele-medicine is a technology that allows doctors to "see" patients without them being in the same place. This is made possible through high-speed communication networks(网络). At both ends of the line, the way of video-meeting lets them see and talk to each other. Blood pressure (血压) and body temperature are directly shown on computers.

    Tele-medicine is already in wide use in some places such as the United States and Germany. Among other uses, village doctors hold "consultations (会诊)"with far-away experts(专家). The experts can also help them during operations.

    Tele-medicine has been slow to reach Asia because of high costs besides other reasons. When things improve, this technology will develop very fast. Chinese University of Hong Kong's medical school has joined a big hospital in Beijing, and Thailand is building a tele-medicine station connecting 20 areas.

    Tele-medicine will do good to patients because doctors from different places can see them and give their medical advice. The help is quite important if the disease is an unusual one. It also helps spread medical knowledge among experts.

    As tele-medicine develops well, its costs will be lower and it will become very popular in the medical field.

    Chinese scientists called on all over the country to fight against serious water pollution in China yesterday.Following days of heavy haze(雾霾)that influenced most parts of China earlier this year polluted water is becoming another big worry among people on popular social Internet

    Websites have called for cutting down water pollution in necessary ways.

    “We should not only protect water,but also pay attention to water pollution,”said Xue Manzi,a well-known investor(投资人)on Sina Weibo.“Dead pigs are found in rivers and waster is poured into rivers and underground.”said Xue.The Huangpu River provider 22 percent of Shanghai's drinking water.But earlier this momth,thousands of dead pigs were found in it.This made people scared and worried,although local government tried to showthat the water was safe.

    In February,some factories in Weifang in Shandong had put waste water underground,polluting the local drinking water.“Clean water and air are necessary for people to live. A responsible government will strictly protect the environment from being polluted,”said a microblogger.

    Isn't it time for us to have a better thinking about our living environment?Do we want to kill ourselves by serious pollution? All the companies should have a serious sense of responsibility.The health of people is far more important than money!