it's my life

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, Little Johnny 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, Little Johnny is on his way to school the next day when he sees the mailman at his front door. Little Johnny 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."