Men have always wanted to fly like birds. Birds can fly easily because they are light, but men's bodies are heavier.

    Men first went up into the air in balloons. These were big bags, and they were filled with gas. Hydrogen (氢气) is a useful gas for balloons. It is lighter than air. Helium is also lighter than air, but it costs a lot of money. So balloons are usually filled with hydrogen.

Balloons have to fly with the wind as they have no engines (发动机) to drive them. Later, men made airships. They are balloons with engines. They are also filled with hydrogen.

    Aircrafts (飞机) with wings now take people across the world. Powerful engines drive these machines across the sky. Some of the engines are like the engines of cars, but they are more powerful.

    There is another kind of engine, we call it the jet (喷射) engine. An English engineer invented the jet engine. At the same time, Germans were also building a jet engine. In May 1941, the English engineer's new engine was fixed in an aircraft, and the aircraft flew quite well.

    Jet engines are very powerful. Usually two to four are enough for an airplane, but some big aircraft need six. Anyone in a moving jet plane can feel the power of the engines. Jet planes can travel faster than sound. As a flying jet plane leaves its noise behind, we do not hear it until it has gone.

      Chocolate is one of the most popular foods in the world. It is popular among people, especially among children. Many people choose it as a birthday gift. When a child gets a box of chocolate as a gift, he may feel very happy.

      Chocolate tastes nice. People eat chocolate candy, chocolate cake, and chocolate cookies. They also drink hot chocolate.

      Chocolate is made from cocoa beans(可可豆). Here are some interesting things about cocoa beans.

      In the past，the Mayas and the Aztecs used cocoa beans as money because they were small，easy to carry，and not easy to find. A turkey cost ninety cocoa beans and a small rabbit cost thirty cocoa beans. They also cooked the beans and  mixed(混合)them with water. Sometimes they added corn or hot pepper. They did not add sugar. The drink was very expensive. Only rich people could afford it.

    "How are you?" is a nice question. It's a friendly way that people in the United States greet each other, but "How are you?" is also a very unusual question. It's a question that often doesn't have an answer. The person who asks "How are you?" hopes to hear the answer "Fine", even if the Derson's friend isn't fine. The reason is that "How are you?" isn't really a question and "Fine", isn't really an answer. They are simply other ways of saying "Hello!" or "Hi!".

    Sometimes, people also don't say exactly what they mean. For example, when someone asks "Do you agree?". the other person might be thinking, "No, I disagree. I think you're wrong. "But it isn't very polite to disagree so strongly, so the other person might say "I'm not  so sure. "It's a nicer way to say that you don't agree with someone.

    People also don't say exactly what they are thinking when they finish talking with other people. For example, many talks over the phone finish when one person says "I have to go now. "Often, the person who wants to hang up gives an excuse: "Someone's at the door." "Something is burning on the stove. "The excuse might be real, or it might not. Perhaps the person who wants to hang up simply doesn't want to talk any more, but it isn't polite to say that. The excuse is more polite, and it doesn't hurt the other person.

    Whether they are greeting each other, talking about an idea, or finishing a talk, people often don't say exactly what they are thinking. It's an important way that people try to be nice to each other, and it's part of the game of language.