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...
