Yawning is an involuntary action that causes us to open our mouths wide and breathe in deeply. No one knows for sure why we yawn, but there are some interesting theories. One states that yawning is physiological: we yawn to draw in more oxygen or remove a build-up of carbon dioxide.
Another theory states that we yawn because we are bored, fatigued, or drowsy, while a third suggests that yawning is evolutionary; according to this theory, our ancestors used yawning to intimidate others by showing their teeth, or to signal a change in activity.
The use of yawning as a form of communication would also explain why some people find yawning contagious; 55% of people will yawn within five minutes of seeing someone else yawn. Reading about yawning usually makes people yawn as well. The average yawn lasts about six seconds. All animals appear to yawn, too!
(reference.com)http://www.reference.com/thisday/index.html?m=2&d=21
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