Science Mysteries: Why Do People Yawn?

Science has given humans many amazing devices and useful bits of knowledge. We know so much about our bodies and how to stay healthy, for example. We also know how to make machines and devices that make life easy for us. Scientists dedicate their lives to knowledge, but even with a lifetime commitment, sometimes scientists fail to find all the answers. One mystery that still remains unsolved is why we yawn—what makes us yawn and what purpose it serves.

Recent studies suggest that we yawn when we get tired as a way to encourage our brains. The theory is that yawning helps cool the brain, allowing it to work more efficiently. According to WebMD's article about these studies, when you yawn, your jaw muscles get stretched, increasing blood flow to your neck, face, and head. Also a deep breath during the yawn forces spinal fluid and blood down from the brain. The cool air then cools down these fluids.

Other scientists believe, however, that yawning is a form of communication. This theory attempts to explain why yawning seems to spread from person to person. This kind of behavior is called "contagious yawning" and carries with it a possible social message. Scientists are not yet sure what exactly the message is, but they believe people who have more social empathy are more likely to start yawning when they see someone else yawning.

Scientists will continue to investigate the mystery of the yawn. Is it a leftover behavior from before humans had language? Was it a "Hey, I'm sleepy. Let's go to sleep" message? Or is it really just a way to keep our brains cool? While this behavior still is largely a mystery, one thing is certain: you are probably fighting the urge to yawn right about now.

Which line shows the main idea of the bolded paragraph?

Science has given humans many amazing devices
We know so much about our bodies and how to stay healthy
Sometimes scientists fail to find all the answers
One mystery that still remains unsolved is why we yawn