A boy goes up to his father and says “Dad. For science homework we’ve got to write an essay on the difference between theory and reality. Can you help?”
His father thinks for a moment and says “Go and ask your mother if she’d sleep with the milkman for a million pounds.” The boy goes away and a few minutes later returns and confirms that she would.
“Now go and ask your sister if she’d sleep with the postman for a million pounds.” Again, the boy goes away and a few minutes later returns and confirms that she would.
“So,” says the father “in theory we’ve got two million pounds, right?” The boy nods his head. “But in reality we’re just living with a couple of slappers, aren’t we?”