Why Do I Yawn?我为什么打呵欠？

Everybody yawns—from unborn babies to theoldest great-grandparent. Animals do it， too. But whydo people and animals yawn？ No one knows for sure.But there are many ideas about why people yawn.

One is that when we are bored or tired， we justdon’t breathe as deeply as we usually do. As thisidea goes， our bodies take in less oxygen2 becauseour breathing has slowed. So yawning helps us bringmore oxygen into the blood3 and move more carbondioxide4 out of the blood.

Yawning， then， would be an involuntary reflex5（something we can’t really control） to help us controlour oxygen and carbon dioxide. Sounds good， butother studies have shown that breathing more oxygendoes not make you yawn less. Also， breathing morecarbon dioxide does not make you yawn more.

Another idea is that yawning stretches6 thelungs7 and other parts of your body. It can be a wayto stretch muscles and joints8， increase heart rate，and feel more awake.

Other people believe that yawning is a reflex to help keep lungs lubricated9inside and keeps them from breaking down. So， if we didn’t yawn， according tothis idea， taking a deep breath would become harder and harder—and that wouldnot be good！

But there is one idea about yawning that everyone knows to be true. It seemscontagious10. If you yawn in class， you may notice a few other people will startyawning， too. Even thinking about yawning can get you yawning. How many timeshave you yawned while reading this article？ We hope not many！

从未出生的婴儿到最年长的曾祖父母，每个人都会打哈欠，动物也会。但是为什么人和动物会打哈欠呢？没有人知道确切的原因。但是关于人们打哈欠的原因有很多观点。

一个观点是当我们无聊或疲劳时，我们只是不像平时那样深呼吸。按照这个观点，我们的身体吸入的氧气减少了，因为我们的呼吸变得缓慢了。所以打哈欠有助于我们将更多的氧气带入血液，并将更多的二氧化碳从血液排出。

那么，打哈欠就是一种无意识的反射（这是我们无法真正控制的东西），以帮助我们控制摄入的氧气和二氧化碳。听起来有道理，但其他研究表明，吸入更多的氧气并不会让你少打哈欠。同样，吸入更多的二氧化碳不会让你打更多的哈欠。

另一个观点是，打哈欠会拉伸肺部和身体的其他部位。它可以是一种拉伸肌肉和关节，提高心率，让人更清醒的方法。

其他人认为打哈欠是一种反射，有助于保持肺部滋润，防止肺部衰竭。所以，根据这个观点，如果我们不打哈欠，深呼吸会变得越来越难———那就不太妙了！

但是有一个每个人都知道是正确的关于打哈欠的看法，就是打呵欠似乎会传染。如果你在课堂上打哈欠，你可能会注意到其他一些人也会开始打哈欠。甚至一想到打哈欠，你就会打哈欠。看这篇文章的时候，你打了多少次哈欠呢？希望不是很多！

（英语原文选自：kidshealth.org）