Why do people yawn?

Yawns are contagious, so I yawn to see how many others will follow

to expell excess carbon dioxide in the body...

People yawn when they need to change their posture.

We yawn when we are tired,we breath less,resulting in increased amounts of carbon dioxide and decreased amount of oxygen in our system. Yawning-opening our mouths wide and taking in bigger breaths in air serves as a way to gather more oxygen into the system. We yawn when we are getting bored and trying to do something different than the present activity!!!