阅读理解
What is the first thing you want to do when you see someone yawn? You want to yawn, too! Yawning is
contagious.
Scientists have found that all people yawn. Babies start to yawn even before they are born. The earliest yawn takes place when a baby is still inside its mother. When young children are about one year old, they start to yawn when they see others yawning. That's why scientists think yawning is contagious.
It's no secret that yawning is contagious, but why do we yawn? Well, scientists are not really sure.
Some think we yawn because we feel bored. Others believe yawning has something to do with breathing. When there is too much carbon dioxide(CO
2) in our bodies, our bodies will tell the brains to breathe deeply. That causes a big yawn.
In 2007, scientists said people yawned to cool down their brains. These scientists found people yawned more often if they were warmer than others.
Scientists are still trying to find out why yawning happens and why it is so contagious. But they are only sure that when one person yawns, immediately everyone else nearby wants to yawn, too.
