At school, Little Johnny learned from a classmate that most adults hide dark secrets, making them easy to blackmail by saying, "I know the whole truth."
Eager to test this, Johnny went home. When his mother greeted him, he said, "I know the whole truth." She quickly handed him $20, whispering, "Just don't tell your father."
Pleased, Johnny waited for his father. Upon his return, Johnny repeated the phrase. His father promptly gave him $40, saying, "Please don't say a word to your mother."
Very pleased, Johnny was on his way to school the next day when he saw the mailman at the door. He greeted him with, "I know the whole truth." The mailman immediately dropped the mail, opened his arms, and said, "Then come give your daddy a great big hug!"