Answers

2015-01-05T22:23:44+05:30
The scientific reason is that when blood supply becomes less the jaw muscles strech, and increse the blood flow towards the head.  
            The deep intake of breath during a yawn forces downward flow of spinal fluid and blood from the brain.
     Cool air breathed into the mouth cools these fluids.
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  • Brainly User
2015-01-05T22:24:36+05:30
When we are tired our lungs don't work properly, so our body receives less oxygen. Then the brain sends signals to yawn or to widely open mouth.
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