Why do we yawn(打哈欠)? Well, scientists are not really sure. Some think we yawn because we feel bored. Others believe yawning has something to do with breathing(呼吸). When there is too much carbon dioxide(二氧化碳) in our bodies, our bodies will tell the brains(大脑) to breathe deeply. That causes a big yawn. In 2007, scientists said people yawned to cool down(使冷静下来) their brains. These scientists found people yawned more often if they were warmer than others.
Scientists are trying to find out why yawning happens and why it is so contagious(可传染的). But they are only sure that when one person yawns, everyone else nearby wants to yawn, too.
s confident(有信心的) that you know sth. or that you are right
w to have a desire(渴望) or a wish for sth.
Some think we yawn because we feel bored.
Some think we yawn feeling bored.
