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 real father a big hug."