ChatterBank1 min ago
Yawn
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Why do we yawn when were tired?
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I don't notice much inrush of air when I yawn, not like deep breathing. And why is yawning 'infectious'? Laughing; you can understand why that can be infectious - it's fun. I think we yawn (like a snarl) as a kind of defence expression due to vulnerability of fatigue but that is a bit thin...
Click http://www.msnbc.com/news/205574.asp?cp1=1
and scroll down to the middle of the page and the paragraph which opens: "The truth is..." for the answer.