根据短文内容，选择最佳答案。①Do you yawn? There are about 20 reasons that scientists think are possible for yawning after research. They do not know exactly why yawning happens, but they do know many facts about yawning.
②We know that everyone yawns in the same way. First you open your mouth slowly. Your mouth stays open for about five seconds. You take in a lot of air and then push it out. Then you quickly close your mouth. We also know that yawning is contagious. When you see someone yawn, you yawn, too.
③Many people say they yawn because they are bored or tired. That might be true. People do often yawn before they sleep and after they wake up. However, we know that people also yawn when they are excited or nervous. Olympic runners, for example, often yawn before a race. Why is that? In 2007, scientists found that a yawn can help a warm brain cool down.
④Some scientists believe that yawning makes you notice things more quickly. When you yawn, you breathe deeply. You also stretch（强化）muscles（肌肉）in your face, mouth, and neck. Your eardrums stretch, too. Maybe this helps you to be quicker to notice things. That's why we yawn.
⑤In some countries, people think yawning is not nice. People put their hands over their mouths to cover a yawn. In other countries, people think yawning is healthy. They think that opening the mouth very big can let good things in. When they breathe out, they think that bad things go out of the body with the air.
⑥Scientists do not spend much time studying yawning. That is probably because yawning does not hurt. It is just something we do.
