Every object has weight. But the center of its weight is not always in the middle of the object. If the top of an object is heavier than its bottom, it will fall over easily. We say that its center of gravity is high. But if the bottom is heavier, it won't fall easily. Then we say that its center of gravity is low.
    Look at the first picture. This empty box must fall over, because its center of gravity (C) is not above the line of its bottom (AB).
Now look at the second picture. There is some heavy iron in the bottom of this box. It will not fall, because its center of gravity is above its bottom.

    Many animals are in danger of dying out. We can see the number of animal species decreases faster and faster and this trend will continue. From 1980 to 2010, at least I million animal species disappeared. Worse still, more and more wild animals are in great danger. It is not a piece of terrible news; it is a fact, a harsh reality. Unfortunately, we may not see these animals in the near future.

    Why is the number of animal species declining year by year? Apparently animals have become victims of fashion industry. Animal skin has been used to make fashionable clothes and these clothes are sold at a high price. So some greedy people begin to kill animals in a large quantity.  This irresponsible behaviour not only breaks the balance of nature but also endangers the living environment of human beings.

    As far as I am concerned, something must be done to stop this illegal action. We believe "no buying, no killing". First, we must make concerning laws to protect these animals in danger.  Second, we must take some measures to protect animals effectively. Animals are our friends and part of our environment. Third, we should raise people's awareness to protect animals and our environment.  In this way, we can build a harmonious society and ensure a sustainable development.

    If there is no rain, there will be no water to drink. We can only live for about three days without water. If there is no rain, we'll have no water to brush our teeth. We will have no water to take a shower. We will have no water to wash dishes or our clothes.

    Plants and animals need water to grow. If there is no rain, they will die. We will not have any rice, any vegetables or any meat. If there is no rain, we will have nothing to eat. We can only live for about one or two weeks without food.

    If there is no rain, there will be no water in the streams or rivers. Fish will have nowhere to live. They will die. We will have nowhere to swim.

    If there is no rain, the cooks in the restaurants will have no water to cook food.

    If there is no rain, firemen will have no water to put out fires.

    When you are waiting for a bus at a station, a 31-metre-long bus is coming to you. You must be surprised. In fact, the 31-metre-long bus is the longest bus in the world. And the bus comes from Germany. An ordinary bus is only about 10 metres long and it can carry only 80 people once. But the longest bus can carry 256 people. So we will need fewer buses in the future. The longest bus can save energy, too. That's to say, it will take less energy than an ordinary bus. Moreover, the driver can drive the longest bus as easily as an ordinary one.

    However, firstly, it has to spend $ 10, 000, 000 buying the longest bus. Someone jokes that we can buy a lot of ordinary buses with that money. Secondly, people think that it is hard for the longest bus to travel on the bustling streets. It may cause new traffic problems. Thirdly, it is easy for someone to get onto the bus without buying a ticket.

Canada Day

Since 1982, July 1st has been officially known as Canada Day. Canadians of all ages take part in this festival across the country.

Canada Day is the year's biggest national party. In many towns and cities, the government organizes a lot of events, often outdoors. These include parades (游行) , concerts, festivals, firework displays and ceremonies for new Canadian citizens (公民) .The celebrations often have a patriotic mood. Canada's national flag, a symbol for Canada Day, can be seen everywhere and a lot of people paint their faces red and white, which are Canada's national colors. The celebrations in Ottawa, Canada's capital city, are especially grand and exciting.

In the province of Quebec, many home rentals (租赁) start on July 1st and last for exactly one year, and many people in Quebec spend Canada Day moving from one house to another. So in Quebec, Canada Day is also known as Moving Day.

Many organizations, businesses and stores are closed this day. Only some bookstores, hospitals and gas stations may be open. Post offices are closed, too. As Canada Day falls in the Canadian summer holiday period, all schools are closed as well. In some areas, special services are provided for large events. The concerts, parades and festivals may cause some traffic jams.