Why do we Yawn? And how can we control it?

Why do we Yawn? And how can we control it?
An indication of tiredness, stress, over-work, lack of directed attention or boredom.
An goings-on indicating psychological decompression after a state of high alert.
A resources of expressing powerful emotions approaching anger, apathy, apprehension, remorse or tedium.
In amalgamation to the above answer they say when your body lacks oxygen you tend to yawn as a rapid way to recieve oxygen.
its because we do not bring back enuff air when we breathe...