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 $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 aword 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."