At school, Little Johnny was told by a classmate that most adults are
hiding at least one dark secret, and that this makes it very easy to
blackmail them by saying, "I know the whole truth."

Little Johnny decides to go home and try it out. He goes home, and as
he is greeted by his mother. He says, "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, "Then come give your daddy a great big hug!".