Why do we open our mouths to yawn properly?

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.

A yawn is a reflex of simultaneous inhalation of air and stretching of the eardrums, followed by exhalation of breath. Pandiculation is the term for the act of stretching and yawning simultaneously.
Yawning is associated with tiredness, stress, overwork, lack of stimulation, or boredom. Yawning can also be a powerful non-verbal message with several possible meanings, depending on the circumstances. In humans, yawning has an infectious quality (i.e., seeing a person yawning or just thinking of yawning, can trigger yawning) which is a typical example of positive feedback.

Hypothesized causes of yawning
1.The shallow inhalation during a yawn is a means of preventing alveolar collapse.
2.The deep inhalation while yawning stretches type II alveolar pneumocytes, which release the surfactant dipalmitoylphosphatidylcholine (DPPC) into the layer of fluid on the alveolar surface.
3.A means of cooling the brain.
4.An action used as an unconscious communication of psychological decompression and stress after a state of high alert.
5.An excess of carbon dioxide and lack of oxygen in the blood.
6.A way of displaying (or indicative of) apathy or arousal.
7.Tiredness.
8.A means of equalizing middle ear pressure.
9.Need of food/drink or hunger/appetite due to reduced level of glucose supplied to the brain.
10.To cool the body, such as after repeated exercise.
11.To moisten and lubricate the sclera and cornea of the eye, or stimulate the tear ducts.
12.To purge the lymphatic system before or after sleep.

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