Though scientists have put forth various theories for yawning—from fatigue to lack of oxygen—none have held up to scrutiny.

"We can put a man on the moon, but we do not understand what the function of yawning is," said study co-author Gary Hack, of the University of Maryland School of Dentistry in Baltimore.

Now, Hack and co-author Andrew Gallup, of Princeton University, propose that yawning causes the walls of the maxillary sinus to expand and contract like a bellows, pumping air onto the brain, which lowers its temperature. Located in our cheekbones, the maxillary are the largest of four pairs of sinus cavities in the human head.

Like a computer, the human brain is "exquisitely" sensitive to temperature and must