At school, little Johnny's classmate tells him that most adults are hiding at least one dark secret, so it's easy to blackmail them by saying, " I know the whole truth. " Little Johnny decides to go home and try it out.
Johnny's mother greets him at home, and he tells her, " I know the whole truth, " His mother quickly hands him £20 and says, " Just don't tell your father " Quite pleased he waits for his father to get home from work, and greets him with, " I know the whole truth, " The father promptly hands him £40 and says, " Please don't say a word to your mother. "
Very pleased, the boy is on his way to school the next day when he sees the postman at his front door. The boy greets him by saying, " I know the whole truth, " The postman immediately drops the mail, opens his arms, and says, " Then come give your Daddy a great big hug! "