Yawning

Why do we have to cover our mouth when we yawn. We don't cover it when we sigh and that's the same air coming out as when we yawn. We don't cover our mouths when we our out of breath and yep you guessed it, same air. And we can't be doing to be polite or to hide the fact that we're yawning because as soon as you cover your mouth, everyone in the room knows you're yawning...people don't randomly cover their mouth unless they're Seelie from the Color Purple.....