"How are you today"? is a familiar question at our local supermarket.
My standard response is, "Oh I have felt worse, but I can't remember when".
This usually results in the cashier giving me a strange look.
My other standard response (but not to the young girls).
To the expression, "See you later" I answer, "Ok, what time".
Usually I get a grin when the person realises what they have said.