Manners are a funny thing. To some, they seem instinctual. To others (especially in Hong Kong), they don't even exist.
Take covering your mouth when you yawn as an example. Apparently, this manner was created in the medieval times in order to prevent the soul from escaping the body (no idea what happened if you opened your mouth to talk or eat).
Later, it became a practical thing. Back when people had horrible dental hygiene (read: none), it was a good way of shielding your friends and loved ones from your personal cloud of rotten breath.
These days, those original reasons may no longer apply, but I can still think of one very good reason to cover your mouth when you yawn.
This is how you look normally:
This is how you look when you yawn:
Now imagine turning a corner and seeing the yawning version of yourself for just a split second, and that's all you'll ever see. Great first impression, no?
Besides scaring the shit out of someone (who thinks you're about to attack), you also resemble a cross between a screaming baboon, shrieking zombie and wild banshee, not to mention that everyone can see right into the depths of your mouth (and anything you left there from lunch).
So, think twice before you go around with your yawn face, ladies and gentlemen, unless that's how you want to be recognized.