Yawn

[Deore]

Why do we yawn when were tired?

[DBusby]

To increase the oxygen content in your blood.

[xyzzyplugh]

Was wondering if that old stock answer would come up :-) 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...

[Quizmonster]

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.

[katiesara]

My boss asked me the other day why yawning was infectious, and wanting to appear knowledgeable I made up the story that it went back to cavemen sharing a cave, and when they saw someone yawning they too yawned in order to maintain their share of oxygen!! Well it wasn't bad for spur of the moment!!

[Stav]

You also yawn when you are hungry