At school, a boy was told by a classmate that most adults hide at least one dark secret, making them easy to blackmail by saying, "I know the whole truth."
The boy decides to try it. At home, he greets his mother with, "I know the whole truth." She quickly hands him $20, saying, "Just don't tell your father."
Pleased, he waits for his father. When his father arrives, the boy repeats, "I know the whole truth." His 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 door. He greets him with, "I know the whole truth."
The mailman drops the mail, opens his arms, and says, "Then come give your father a big hug."