Bacteria
When you leave grains in your mouth for long enough, they'll turn into flowers of bacteria. These aren't the nice, pretty flowers you'd give your mom on Mother's Day. These are little bacteria flowers that are taking over your mouth. Maybe brush your teeth next time.
Imagine how much bacteria would flower in your mouth if you just left grains in there for like, I dunno, a month. That's a lot of nasty bacteria!
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There is a lot of this bacteria in the King (きん) of England's mouth. He eats a lot of grains, and he won't let anyone touch his mouth to clean it, but he refuses to do things himself... so it's absolutely chock full of bacteria.
Picture the King's mouth up close. Look at all the bacteria squiggling around and through his teeth and gums. Bleh! Barf!