Have you ever wondered why birds sing? Maybe you thought that they were just happy. After all, you probably sing or whistle when you are happy.

    Some scientists believe that birds do sing some of the time just because they are happy. However, they sing most of the time for a very different reason. Their singing is actually a warning to other birds to stay out of their territory.

    Do you know what a 'territory' is? A territory is an area that an animal, usually the male, claims as its own. Only he and his family are welcome there. No other families of the same species are welcome. Your yard and house are your territory where only your family and friends are welcome. If a stranger should enter your territory and threaten you, you might shout. Probably this would be enough to frighten him away.

    If so, you have actually scared the stranger away without having to fight him. A bird does the same thing. But he expects an outsider almost any time, especially at nesting（筑巢）season. So he is screaming all the time, whether he can see an outsider or not. This screaming is what we call a bird's song, and it is usually enough to keep an outsider away.

    Birds sing loudest in the spring when they are trying to attract a mate and warn others not to enter the territory of theirs.

    You can see that birds have a language of their own. Most of it has to do with attracting mates and setting up territories.

    Height is just one of the features your genes(基因) decide. In fact, because you have two parents, your genes provide you a height that usually lands somewhere between the height of each parent. If both your parents are tall, then most probably you will be tall, too. But if you have questions about how tall you're going to be, ask your doctor if he or she can help you find them out.

    But genes don't decide everything. For example, eating an unhealthy diet can keep you from growing to your full potential(潜力). Getting plenty of sleep and enough exercise will help you grow to the expected height.

    It's natural that you're wondering how fast you should grow. There's no perfect or right answer. Generally speaking, kids grow about 6 cm a year between age 3 and the time when they start puberty.

    Your doctor will know how your growth has been going over the years. 2 cm here and 3 cm there make a little difference to the height you're at now. How have you been growing up to this height? What other changes may your body be going through?

    Don't be amazed if you seem to have grown a lot in a very short time. Everyone has a growth spurt(高峰) during puberty. The age for starting puberty is about 10 for girls and about 11 for boys. But it can be earlier or later — between 7 and 13 for girls and 9 and 15 for boys.

    You'll usually begin to notice that you're growing faster about a year after your body starts to show the first changes of puberty.

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    Radios, telephones and televisions are widely used in the world. When you switch on the radio, you can listen. But when you use the telephone, not only can you listen to others, but also you can chat with them. However, you cannot see anything at all. Television is much better than both of them. People can watch TV and listen to it, but they cannot take part in what they see.

    Today, some people are using a new type of telephone called vision phone. With it two people who are talking can see each other. Vision phones can be of great use when you have something to show the person whom you are calling. It may also have other uses in the future. Some day you may be able to call up a library and ask to read a book right over your vision phone. You may be able to do the shopping through it too. Perhaps in the near future, vision phones will come into our everyday life.

    Cars make our lives more convenient. But they can also be a problem. For example, fossil fuel(化石燃料) cars mainly cause air pollution.

    To improve the environment, many countries are developing electric vehicles (EVs,电动车). Among them, China is taking a leading role and has created the world's largest EV market.

    Last year, over 40 percent of the 753,000 EVs sold in the world were sold in China, more than twice as many as the number sold in the United States.

    "Filling up a fossil fuel car with gas(汽油) is more expensive than charging( 充电) an electric car，" said Wu Hao，who bought an electric car this year.

    However, there are still some problems. It can be hard to find charging piles(充电桩) and the cars can't go far before needing to be recharged.

    Some of these problems are being solved. In 2014，China had 31,000 charging piles. Now China has the world's largest EV charging network, with more than 167, 000 charging piles in total.

    Developing EVs is one of many efforts that China has made to cut down pressure on its environment after the government promised to stop increases in carbon dioxide emissions(二氧化碳排放量) by 2030.

    "As China goes, so will the world's car industry," The Wall Street Journal said. China has taken a leading role in the world's electric vehicle industry.

Whenever we think about something, most of us have a feeling of words going through our minds. However, if you stop and try to listen to your own thoughts, you'll notice that they are not in order at all. It's easy to imagine that we think in the language that we speak, but this is completely wrong. Language can show the results of our thinking, but it's not the thinking itself. 

Getting the joke

Think about what happens when you "get" a joke: "I've decided to sell my Hoover (吸尘器). . . well, it was just collecting dust (灰尘). " Whether you laugh or not, you react (反应) at once. Then you try to put into words what the joke actually is. 

Another example is the thinking you need when playing a fast-moving video game. You might shout "Yes!"or "Got you!". But if you had to talk about all your plans aloud, your game play would slow down. Thoughts always come first and the expression of our thoughts in words comes later. 

Thinking in silence

Humans think about things without language all the time, and only report their thoughts in the language when they have time to do so. You can test this with a game. Read a word quickly, out loud or in your head. You'll find that while you're doing this, you won't be able to think with words, but you can still plan, reason and imagine, pretty much as normal. 

Smart minds

Just because humans don't need language to think, it doesn't mean that words don't help us. The real magic of language is that it allows people to share their thoughts with others. This means you don't have to face the world by yourself-you can learn from ideas created by the elderly. This lets humans develop science and technology, laws and stories. People can think without language, but language makes humans so smart. 

 This week, I asked my classmates about their vacations. Here are some of their answers.

 Linda went to Sydney, Australia. The weather was sunny and the people were friendly to her. She went there by plane and the air trip was relaxing. She went to Sydney Opera House. It was wonderful. She had a great vacation.

 Alice went to visit her aunt. At first the weather was rainy and wet. She had to stay at the house. She watched TV but the TV shows were boring. Later on, the weather got better. So she went shopping with her aunt. The shop assistants were all friendly and kind. She bought something nice for herself.

 Peter stayed at home. First he did his homework. It was a little difficult. Then he played computer games. They were interesting. He thought his vacation was OK.