And now you know by following the link:
Why We Yawn—And Why Yawning Is So Contagious
“Scientists haven’t nailed down the exact cause of our 240,000 lifetime yawns yet, but there’s good support for the yawn as a temperature regulator. In 2007, researchers at the University of Albany showed students a video of people yawning. Half the students were instructed to breathe through their nose, and the other half were told to in and exhale through their mouths. Of the mouth-breathing group, half of them yawned at the video, but among nose breathers, yawning was pretty much non-existent.
It could be that the mouth-breathers were overheating their gray matter. Because the brain burns up to a third of our daily caloric intake, it generates a bit of heat. And our brains work better when they’re not too toasty.”
Did you know snakes and fish yawn? And that yawns are contagious?