why do we yawnWhy and when do we yawn? Why yawning in contageous?

What is yawn? What is the origin of a yawn? Does yawning helps the body?
It seems that yawing is way of getting more oxygen into your system.
Sorry Rohn, may be it was judged as the best but I am not happy as it is not a complete answer. The complete answer is expected to define the process of yawn from its origin in body & mind and up to the end, i.e. inhaling of more oxygewhy do we yawnn and also why it is contageous.

Yawning is in response to low oxygen levels.
It's contagious because you taking out oxygen > decreased oxygen levels in air > other people have low oxygen levels > other people yawn.

-yawn-
Yawning is a normal response to fatigue and drowsiness, but excessive yawning can be caused by a vwhy do we yawnasovagal reaction. This reaction is caused by the action of a nerve, called the vagus nerve, on the blood vessels. It may indicate a heart problem.

Normal yawning may be induced when the brain or lungs are not getting enough oxygen (or when someone else yawns).

Drowsiness or weariness disorders associated with excessive daytime sleepiness,may cause yawning too.