Yawning is a mostly involuntary process of opening the mouth and breathing in deeply, filling the lungs with air. It is a very natural response to being tired. In fact, yawning is usually triggered by sleepiness or fatigue. Or boredom. Notice that when you get up from a good sleep you never yawn. Infact, it is difficult to yawn. But there is an exception and this happens to yours truly with regular frequency. The setup is very simple. I'm fresh from a 10 hour sleep ( I literally am a 19 year old baby when it comes to sleeping) . I freshen up and my father calls me to show something in the stock market. Within 30 seconds of him speaking, I give one big yawn. If I sit for another 10 minutes, guaranteed 5 more yawns. So much so that my jaws ache. Now at this point if I start reading newspapers, look at my phone, or do something else, the yawning vanishes. Now if he calls me again to see the charts, the yawn comes again. How do you explain all this ?