What is yawn and why people do that?

Q. What is yawn and why people do that?
A. Many people believe that yawning is caused by low oxygen in the lungs. This is a myth.
About real explanation of the yawn you can find at http://en.wikipedia.org/wiki/Yawning and http://www.wrongdiagnosis.com/sym/yawning_excessively.htm

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