Why Yawn?

Why do we yawn? We are tired so we yawn?

The average duration of a yawn is about 6 seconds.

When we are bored or tired, we just don't breathe as deeply as we usually do. As this theory goes, our bodies take in less oxygen because our breathing has slowed. Therefore, yawning helps us bring more oxygen into the blood and move more carbon dioxide out of the blood.

Dr. Provine suggest that perhaps yawning is like stretching. Yawning and Stretching increase blood pressure and heart rate and also flex muscles and joins. Evidence show that if you try to stifle or prevent a yawn by clenching your jaws shut, the yawn is somewhat "unsatisfying". The stretching of jaw and face muscles is necessary for a good yawn.

I agreed with this as when i saw this article and see the image of yawning, I just cant stop yawning. And normally I dont yawn so many times in a day.

Yawning is possible to be contagious because of a leftover response (a "vestigal" response) that is not used anymore. But I felt it is contagious as when I saw someone yawn. I will yawn.