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yawn; carbon; dioxide; surfactant; oxygen; boredom; communication ;


Why do we yawn?

Well, be prepared to yawn when you are reading this, because one theory about yawning is that it is 'catching'!

  • That means that seeing someone yawn or even thinking or reading about yawning can make you yawn too!
  • Another theory is that we yawn when we have too much carbon dioxide (say car-bon-d-eye-ox-ide) in the blood, and yawning makes us take in more oxygen (say ox-ee-jen). But experiments showed that if the amount of carbon dioxide in the air around people who are yawning was lessened, it didn't make any difference to the people who were yawning. They just kept right on yawning! Even when more oxygen was put into the air, they still kept on yawning!
  • Some people believe that yawning stretches the lungs and lung tissue to give the lungs a quick work–out. Maybe this is why many athletes yawn before they start their races?
  • Some people say that yawning helps to move the surfactant (say surf-act-ant) around in the lungs. This is the oily stuff that coats the lungs inside and keeps the tubes from sticking together so that the air can flow smoothly around all the inside bits of the lungs. (See the topic Your lungs if you want to know more.)
  • Some people believe that yawning cools down the brain if it gets hotter than the rest of your body temperature.
  • Some people say that as animals also yawn, yawning could be a form of communication - like saying, "I don't want to listen any more," or "I'm feeling uncomfortable about this", or "Stop now, I'm getting angry."
  • Finally, some people think that yawning is a sign of boredom.

Are you yawning yet?

What happens when you yawn?

  • yawningWell, first your mouth opens and your jaw drops down.
  • This allows more air to rush into your mouth.
  • That air goes to your lungs.
  • Your diaphragm muscle is pushed right down flat, letting your lungs expand more than usual.
  • The air rushes round the lungs and oxygen is taken out and sent off in the blood vessels.
  • As you breathe out, carbon dioxide is breathed out too.

The section 'Breathing out' in the topic Your lungs, will tell you more about this.

Did you know?

  • Yawning is an involuntary (say in-vol-un-tare-ee) action. That means that you don't tell your brain that you are going to yawn, it just happens. We know this to be true because babies yawn even before they are born!
  • A yawn lasts about 6 seconds.
  • If you are yawning in class then probably over half the class is yawning with you!
  • Scientists say that fish yawn too!
    fish yawning

Some fun stuff to try

pet yawningWatch your pets and see if you can work out what message they are giving when they yawn. It could be a very interesting bit of research, or even good material for a project!

If you are getting bored travelling, try yawning and see how long it takes before you get a whole heap of people joining in.

Try yawning when you are at the zoo watching the animals and see if they start yawning.

Dr Kim says:

Dr KimEverybody yawns without realising at times. Remember that some people may not have read this topic and may think that yawning is a sign of boredom or even rudeness, and they could be offended.

Try to cover your mouth when you yawn, and it is also polite to say "pardon me!" if you yawn when others are around. Have fun with your research and let us know how it went on the feedback button.


I'm sorry that I'm yawning
You must think me really rude.
I'm really interested
And I really like your food!
Maybe I'm just very tired
Or maybe I'm too hot.
No, I don't think you're boring.
Help! I'm yawning! No I'm not.
I'll hold my chin so it can't drop.
Maybe then the yawns will stop.



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We've provided this information to help you to understand important things about staying healthy and happy. However, if you feel sick or unhappy, it is important to tell your mum or dad, a teacher or another grown-up.


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