阅读短文，回答问题。 What do you hear? Sounds all round you! You can hear the sound of traffic, the wind in the trees, a dog barking, your own breathing and lots of other things. But what is sound? Sound happens when something vibrates (振动) or shakes. We can make something vibrate by hitting it. Try this experiment (实验). Put your ruler on your desk so that part of it sticks out (伸出) over the edge. Put your hand on the part of the ruler that is on the desk. Now pull the part that is sticking out over the edge down, and then let it go. The part you let go will vibrate. It will move up and down very quickly, and give out sound. The part of the ruler sticking out over the edge of the desk produced the sound. If you make that part longer, you will hear a lower sound. When you shorten it, you will hear a higher sound. Why does this happen? When you shorten the ruler, it will vibrate more quickly. When something vibrates more quickly it produces a higher sound. If you lengthen the ruler, it will vibrate more slowly. When something vibrates more slowly, it produces a lower sound. 1. How does sound happen? ________________________________________________________ 2. How can we make something vibrate? ________________________________________________________ 3. Can we hear a higher sound or a lower sound if we make the part of the ruler sticking out over the edge of the desk longer? ________________________________________________________ 4. What does the word'lengthen' mean in Chinese? ________________________________________________________ 5. What's the best title of the passage? _________________________________________________________