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 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."

This is one of the funniest jokes I've ever heard. Thank you for
sharing.