Can you remember a morning when you’ve woken up to the ring of your alarm clock and wished you could stay in bed? You haven’t slept well, your hair is a mess. Worse, you can’t stop yawning. Yawning is universal to humans and many animals. Cats, dogs and fish yawn just like humans do! Yawning is an involuntary(无意识的,不自觉的) action during which you open your mouth and breathe deeply. The average duration of a yawn is about 6 seconds.
The purpose and cause of yawning are still a mystery. Scientists are not sure what causes a yawn. People yawn when they are tired or bored, but they also yawn when they aren’t. There are many ideas explaining why we yawn.
One idea is that yawning is a way for the body to become more alert by taking in more oxygen. A yawn increases the heart rate, forces carbon dioxide out of the lungs and blood stream, and brings oxygen to the brain. But one study showed that volunteers given a lot of oxygen did not yawn any less than before, while those given a lot of carbon dioxide did not yawn more.
Another idea is that early humans used yawning as a form of communication. If one decided it was time to sleep, they would tell the others by yawning and they would do it in return to show they agreed.
A third explanation comes from psychology professor Gordon Gallup of University at Albany in New York. He said that as people yawn, they cool off their brains. “Brains are like computers.” he said. “They only operate efficiently and effectively when they’re cool. Many things connected to yawning, like being tired, make the brain hot, and yawning can reduce the heat.”
Scientists have sent people into space, and created terrible nuclear weapons, but there are lots of seemingly simple things, such as why we yawn, or hiccup（打嗝）, that they can’t figure out. Next time you are in class in the morning, let out a big yawn and watch to see how many of your classmates yawn in response!
What’s the purpose of the description of how bad you may feel when you’re woken up in the morning?

A．To explain why humans yawn.

B．To explain how humans start yawning.

C．To lead into the topic of yawning.

D．To complain about being woken up by the clock.

Which is not among the possible reasons for yawning mentioned in the passage?

A．A way to show that you feel hot.

B．A way to get rid of carbon dioxide.

C．A way to cool off your brain.

D．A way to communicate.

In which situation are you likely to yawn?

A．When you feel frustrated.

B．The weather is hot.

C．Someone else yawns near you.

D．When you breathe in a lot of carbon dioxide.

What can we learn from the passage?

A．The real reason why humans and most animals yawn.

B．Hiccups are an involuntary human action similar to yawns.

C．The more an action can be studied, the easier it is to research.

D．Finding out why humans yawn should be easier than sending people into space.