Why exactly do people yawn?

cause of yawn due lack of oxygen brain makes tired.
reason exact motion of yawn believed date colonial man , how man use lookout each village warn others of danger. motion of yawn seen , replicated throughout village because know yawns infectious , used others know lookout getting tired.

i realized symbolizes you're tired or dying of boredom why yawn? i've been told wakes brain yet feel more exhausted after. eyes begin water. why this?

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