gonna take a guess that this is the joke you might be looking for, but will search further with the clues that you gave
A teenager comes home from school with a writing assignment. He asks his father for help. "Dad, can you tell me the difference between *potential* and *reality*?" His father looks up thoughtfully, and then says, "I'll display it for you. Go ask your mother if she would sleep with Robert Redford for a million dollars. Then go ask your sister if she would sleep with Brad Pitt for a million dollars. Then come back and tell me what you've learned." The kid is puzzled, but he decides to see if he can figure out what his father means.
He asks his mother, "Mom, if someone gave you a million dollars, would you sleep with Robert Redford?" His mother looks around slyly, and then with a little smile on her face says, "Don't tell your dad, but yes, I would." Then he goes to his sister's room and asks her, "Sis, if someone gave you a million dollars, would you sleep with Brad Pitt?" His sister looks up and says, "Omigod! Definitely!"
The kid goes back to his father and says, "Dad, I think I've figured it out. Potentially we're sitting on two million bucks, but in reality, we are living with a couple of whores."
