At school, a boy 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”. The boy
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 a $50 note 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
also promptly hands him a $50 note 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.”