What is the first thing you want to do when you see someone yawn(打呵欠)? You want to yawn, too! Yawning iscontagious.
Scientists have found that all people yawn. Babies start to yawn even before they are born. When a baby is inside its mother, he or she yawns. 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.
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(呼吸). When there is too much carbon dioxide( CO2 ) 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 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. But they are only sure that when one person yawns, everyone else nearby wants to yawn, too.
