Look at the centre circles of Diagram (图)A and Diagram B. Which of the centre circles looks larger? In fact, they are exactly the same size.
Then look at the picture on the right. What do you see? A vase? Or two faces？Does the picture change quickly from one to the other again and again? Maybe or maybe not, but you can never see them at the same time.

“What's happening? Is something wrong with my eyes?” You may wonder at what you see. Don't worry. Here is how it goes:
When we look at things, our eyes send messages to our brains and then our brains interpret（解读）the information. However, sometimes our brains interpret the received information in a wrong way. It seems that our eyes are playing a joke on us. This often happens and we call it “visual illusion(视觉错误)”.
Movie makers often make use of visual illusions. They make the objects around actors much smaller or bigger than usual. This makes us believe that the actors are much larger or smaller than they usually are. The movies Jurassic Parkand Honey I shrunk the kids just make use of visual illusions.

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    Pollution inside homes and other buildings kills more than 4 million people each year. Many people die as a result of breathing smoke or from cooking over wood-powered or coal-powered stoves.

    One way to reduce the number of deaths is through cooking equipment powered by the sun. Crosby Menzies, a solar power expert in a South African company, described his latest solar cooker called the "Sol-4". "It is four square meters of mirrors, six to eight meters in length. It is quite a large cooker. "

    The "Sol-4" works by reflecting (反射) light from the sun off large mirrors. The mirrors direct (把……对准) the light at a cooking pan. Then the light heats the pan with solar energy. Within two minutes, the pan is hot enough to cook sausages and onions. In just four minutes, water can be boiled. That is as fast as cooking with natural gas or electricity.

    The cooker is also much "friendlier" than other models because people do not have to stand in the sun to use it. And people can prepare meals without having to collect firewood or spend money on coal. Such individuals would be less likely to have breathing illnesses from harmful smoke.

    At present, the cooker only works when the sun is shining. But engineers are working on a way to make it work without sunlight.

    Each cooker costs about $ 2,000, which is a large amount of money for most Africans. To solve that problem, Crosby Menzies has created a non-governmental organization called Solar Cookers for Africa.  It plans to raise money through donations to provide poor people with solar cooking equipment.

    Do you think you're green? You're a totally modern citizen, aren't you? You keep your bottles and take them to the bottle bank. You have a black bin and a green bin, one for general waste and one for recyclable waste. Your car uses lead—free petrol. You use the same plastic bag every time you go to the supermarket. Maybe you even cycle to work from time to time. You are playing your part; you are green and environment—friendly. Well, I've got news for you:

    YOU ARE NOT DOING ENOUGH!

    If you don't believe me, look at these facts. Trust me, they're from a BBC website.

    On average every person in the UK throws away their own body weight in rubbish every seven weeks.

    Every year, the average dustbin contains enough unused energy for 500 hot baths, 3,500 hot showers or 5,000 hours of television.

    Every UK household produces over one tonne of rubbish every year.

    If all the aluminium drink cans sold in the UK were recycled, there would be 14 million fewer full dustbins per year.

    Just one recycled aluminium can saves enough energy to run a television for three hours!

    At Christmas over 83 square kilometres of wrapping paper is thrown in the bin.

    Almost three billion nappies (used for baby) are thrown away in the UK every year. 90% of these are buried and take hundreds of years to disappear.

    Have I made you believe that you need to do more? No? OK, here are one or two more facts.

    Recycling just one plastic bottle saves enough energy to power a light bulb for six hours.

    Collecting dustbins and cleaning streets costs ￡1. 6 billion per year.

    Surely you must believe me now. You need to do more. The good news is that there is some— thing else you can do.

    What should you do if there's an earthquake at school? How can you protect yourself when you are caught in a stampede(踩踏)?March 28, 2016 provided you with a good chance to learn about safety.

    This day was called the National Day of Education on the Safety of Elementary and Middle School Students(NDESEMSS)(全国中小学生安全教育日). The theme of the day was to give students more knowledge about safety and make sure their lives are safe.

    A survey done by the China Youth and Children Research Center showed that school accidents kill more elementary and middle school students than anything else. The main accidents come from stampedes, earthquakes, fires and sports injuries(伤害).

    What can we do to keep ourselves safe? Staying calm is the first and most important rule when facing accidents. But different accidents have different self-protection advice.

    A stampede is possible anywhere. The larger a crowd gets, the more likely stampedes become. When students around you begin to push, stand still and try to hold onto something, or stay in a corner until the crowd leaves. If you fall down in a moving crowd, cover your head with both hands. Lean to(偏向) one side, curl up(蜷缩) your body and bend your legs.

    Earthquakes are common natural disasters(灾难). When you feel the ground shake, drop down, take cover under a desk and hold on. You should stay indoors until the shaking stops. If you are outdoors, don't stay near buildings, trees or power lines.

    When there's a fire, follow the teachers' instructions, leave the classroom quickly and use a piece of wet cloth to cover your mouth and nose so that you don't breathe in smoke.

    Bamboo(竹子) is one of nature s most surprising plants. Many people call this plant a tree, but it is a kind of grass.

    Like other kinds of grass, a bamboo plant may be cut very low to the ground, but it will grow back very quickly. A Japanese scientist reported one bamboo plant which grew 1.5 metres in 24 hours! There are more than 1, 000 kinds of bamboo. Bamboo grows almost everywhere in the world, but in Europe, they can't grow.

    Not all bamboo looks the same. Some bamboo plants are very thin. They may only grow to be a few centimetres wide while others may grow to be more than 30 centimetres across. This plant also comes in different colours, from yellow to black to green.

    Bamboo has been used to make many things such as hats and kitchen tools. Because it is strong, bamboo is also used to build buildings.

    Many Asian countries have used bamboo for hundreds of years. They often use bamboo for buildings and supporting(支撑) new buildings and bridges while they are being built.

    In Africa, poor farmers are taught how to bring water using bamboo. These African countries need cheap ways to bring water because they have no money, and their crops often die from no rain and no water. Bamboo pipes(管子) help poor farmers bring water to their thirsty fields without spending much money.