My friend told me that the answer of the riddle was "tail". Can you guess why?
~ When dogs are happy (or smiling), they kind of thump it back and forth. When they're unhappy (frowning) they flick the tip of it back and forth quickly.
~ The wife of a king is a queen, but it refers to the Canadian coin which has a queen on it. The opposite side is tails.
~ A scorpion's stinger is also called a tail.
~ The "up when up" and "down when down" is kind of to throw you off. Tails can be up or down, therefore they're either up or down.
This is one of the best riddles I've heard in a long time.