Sometimes people feel sleepy. They can't stay interested in what they are doing.有时人们会觉得很困。他们对于自己所做的事情无法保持兴趣。That is when they open their mouths wide. We call this a yawn. It makes a pretty silly face.也就是他们张大嘴巴的时候。我们称之为哈欠。这看起来很傻。Why do people do it? 人们为什么这样做?Scientists do not know much about yawns.科学家们对哈欠知之甚少。But they have some ideas. 但是他们有一些想法。Our bodies get tired. 我们的身体会疲惫。Our minds get tired, too. 我们的大脑也会感到疲劳。A tired mind needs air. So we yawn.疲倦的大脑需要空气。所以我们才会打哈欠。The quick yawn takes in air. 快速的哈欠会吸入空气。And the air wakes up our mind. 接着空气会唤醒我们的大脑。But not for long. Watch out! 但是不会太久。小心!Here comes another yawn. 接着又来一个哈欠。
