text: It sounds weird, but I personally think that when you see someone yawn, its kind of like a mental thing to think tiredness. So you start thinking tired, and eventually yawn.\nI'm no expert so don't like hold me to this. =)\n\nI do know that if you hold open you're mouth for a long time, you eventually yawn. I have no clue why I just heard it somewhere. So you might want to ask that in another question haha.