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。（剩余507字）