Starter check

Part B — Passage 2: "Why Do We Yawn?"

(1) Everyone yawns, but scientists still do not fully understand why. (2) For a long time, people believed that yawning brought extra oxygen into the body. (3) That idea has been mostly abandoned, because studies show that breathing extra oxygen does not reduce yawning. (4) A newer idea is that yawning cools the brain. (5) When you yawn, you draw in a large breath of cool air, and the muscles around your jaw stretch and increase blood flow to the brain. (6) The cooler blood may help the brain work better, especially when you are tired or bored. (7) This could explain why we yawn more in warm rooms and less in cold ones. (8) Another idea is that yawning helps us stay alert. (9) A yawn stretches the muscles of the face and neck, and the deep breath increases heart rate slightly. (10) This may be the body's way of waking itself up when attention is fading. (11) Yawning is also contagious, and seeing someone else yawn makes most people yawn too. (12) Scientists think this may be linked to empathy, because people who score higher on empathy tests tend to yawn more often in response to others. (13) My dog yawns every morning when I wake him up for a walk. (14) Whatever the reason, yawning is one of the few behaviors that humans share with almost every other animal with a backbone. (15) Fish yawn, birds yawn, and even snakes yawn. (16) The next time you catch yourself yawning in class, remember that your brain may simply be trying to cool down and wake up. (17) It is not a sign that you are bored with the lesson, although your teacher may not believe that. (18) The science of yawning is still unfolding, and researchers continue to test new ideas.