Joke
 

A boy gets an assignment at school to determine the difference between "theory" and "in-practice" He's unable to wrap his mind around the concept, so he asks his father for some assistance.

The dad tells the boy: "Go ask your mother if she'd sleep with Robert Redford for a million dollars".

The boy does as he was told, and asks his mother, who replies, "Absolutely I would!"

He returns to his father who then instructs him, "Go ask your sister if she'd sleep with Brad Pitt for a million dollars."

Again the boy complies and asks his sister, who replies, "Hell Yeah! In a heartbeat".

So he returns to his father with this information.

The father says, "Listen carefully now: In theory, we live with two millionaires. In practice, we live with a couple of common whores."