At school, a boy is 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" even when you don't know anything.
The boy decides to go home and try it out. As he is greeted by his mother at the front door 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 drops the mail, opens his arms and says, "Then come give your FATHER a big hug!"