At school, Little Johnny's classmate tells him that most adults are hiding at least one dark secret, so it's very 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, the boy 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 mailman at his front door. The boy greets him by saying, "I know the whole truth."
The mailman immediately drops the mail, opens his arms, and says, "Finally! Now come give your old man a great big hug!"