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Definition of Yawn

Yawn: Involuntary opening of the mouth, accompanied by slowly breathing inward and then outward. Repeated yawning can be a sign of drowsiness, boredom, or depression. The yawn helps to open tiny air sacs (alveoli) in the lungs that can collapse during shallow breathing.

Source: MedTerms™ Medical Dictionary
http://www.medterms.com/script/main/art.asp?articlekey=6047
Last Editorial Review: 3/19/2012

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