Bev could not do her work. She felt too tired. She put down her book, yawned (打哈欠) and felt better. Bev looked up. Had the teacher seen her? To her surprise, the teacher was yawned, too. He took a long breath (呼吸) and opened his mouth wide. He closed his eyes a bit and let out his breath in a big, slow yawn. Then Bev heard other yawns.

The teacher said, “Bev yawned because our doors and windows are closed. And the room is warm. Her body needed more air. Maybe she yawned because she was tired. Maybe she was hungry. Or she just needed to move a bit. Do you feel better now, Bev?” he asked.

“Yes, “ said Bev.

“Yawns wake us up,” the teacher said, “When you yawn you stretch (伸展) many parts

of your body. that stretching makes you feel good.” There were more yawns. “People do not know why they yawn,” the teacher said. “But if you see a yawn, hear one, or even read about one, you may yawn too. And once your yawn starts, you can’t stop it. You may close your mouth to stop it. But the yawn will still come.”

The teacher stood up and said, “Let’s open the window and let some air in. You may also go out for a drink. A cold drink or water on your face help to stop yawns. Bev may go first.”

How do you feel now after you have read this story? Did you yawn? Watch a friend read this. See if your friend starts to yawn. If you see a yawn, you will know why!

The cause of yawning is ______.
A. the room is airless B. the room is warm

the answer is A
引起哈欠的原因有很多，针对这篇文章的话原因是因为教室的窗户紧闭，通风不良，二氧化碳的增加所导致的。