I'll apologize first--because I know this will make you yawn!

Why do we yawn?? That popped into my head again last night, as it has for some time now.  And why is it contagious?  Seeing someone else yawn makes us yawn, too; and even thinking about it makes me yawn! Here's the explanations I found:

One is that 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.

Another thought:

A yawn is an involuntary reflex where the mouth is opened wide, and the lungs take in a lot of air. The air is then exhaled slowly. During this time, the eardrums stretch, and the eyes may also close tight, causing them to water.

No thought or action has to be taken to produce a yawn, and the process is similar for everyone. Yawning commonly occurs either before or after sleep, which is why it is usually considered a sign of being tired. Yawning also occurs frequently in people who are doing boring or tedious things.

There is also a social aspect to yawning. Yawning appears to be contagious among humans and other animals, and the contagiousness of a yawn is well documented but hardly understood. 

On a personal note: I tend to yawn if I'm nervous! That can be embarrassing, since whoever I'm with probably thinks I'm bored wih the conversation.