A new study suggests there's science behind your yawns.
Opening wide and taking it all in cools the brain and regulates its temperature for optimum functionality.
But there's a window: People yawn most when it's around 68 degrees. When it's really hot outside, people are less likely to yawn because it would have very little impact on the brain's temperature.
As for how much sleep you got the night before or how stressed out you are, those are factors that could cause the brain to overheat, thus the necessity for a nice big yawn.
As for the phenomenon of contagious yawning? Science has yet to figure that one out.
Speaking of yawning, celebs do it too -- and it's a lot funnier than it should be: