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！

從未出生的嬰兒到最年長的曾祖父母，每個人都會打哈欠，動物也會。但是為什么人和動物會打哈欠呢？沒有人知道確切的原因。但是關(guān)于人們打哈欠的原因有很多觀點。

一個觀點是當(dāng)我們無聊或疲勞時，我們只是不像平時那樣深呼吸。按照這個觀點，我們的身體吸入的氧氣減少了，因為我們的呼吸變得緩慢了。所以打哈欠有助于我們將更多的氧氣帶入血液，并將更多的二氧化碳從血液排出。

那么，打哈欠就是一種無意識的反射（這是我們無法真正控制的東西），以幫助我們控制攝入的氧氣和二氧化碳。聽起來有道理，但其他研究表明，吸入更多的氧氣并不會讓你少打哈欠。同樣，吸入更多的二氧化碳不會讓你打更多的哈欠。

另一個觀點是，打哈欠會拉伸肺部和身體的其他部位。它可以是一種拉伸肌肉和關(guān)節(jié)，提高心率，讓人更清醒的方法。

其他人認(rèn)為打哈欠是一種反射，有助于保持肺部滋潤，防止肺部衰竭。所以，根據(jù)這個觀點，如果我們不打哈欠，深呼吸會變得越來越難———那就不太妙了！

但是有一個每個人都知道是正確的關(guān)于打哈欠的看法，就是打呵欠似乎會傳染。如果你在課堂上打哈欠，你可能會注意到其他一些人也會開始打哈欠。甚至一想到打哈欠，你就會打哈欠?？催@篇文章的時候，你打了多少次哈欠呢？希望不是很多！

（英語原文選自：kidshealth.org）