A kid 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 to 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 father, 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 are sitting on two million bucks, but in reality, we are living with two sluts."