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."