Why do we open our mouth to YAWN...

A yawn is a reflex of simultaneous inhalation of air and stretching of the eardrums, followed by exhalation of breath. Yawning is commonly associated with tiredness, stress, overwork, lack of stimulation and boredom, though recent studies show it may be linked to the cooling of the brain. In humans, yawning is often triggered by others yawning (e.g., seeing a person yawning, talking to someone on the phone who is yawning) and is a typical example of positive feedback This "infectious" yawning has also been observed in chimpanzees and dogs.
         The involuntary act of yawning usually includes opening the mouth very wide while slowly taking in a deep breath. This contortion of the mouth puts pressure on the salivary glands causing the eyes to sometimes tear-up, causes the throat to better open, and tightens the muscles around the mouth making sure the yawn is better accomplished.