Ask your parents.
Nobody asks, 'who's there ' when you try and tell a knock knock joke.
You ask him politely.
The parents would love to know.
If Boy is in love - His parent Ask: Idiot, Who is that Girl.. Moral :: No matter who ever is in love... Boys Are Always Idiot :P (LOL)
I charge five dollars if its a boy and five dollars if its a girl. Lets just say this ones on the house.