the difference between Potentially and Realistically?

This little boy goes up to his dad and he says "Dad?, What's the difference between Potentially and Realistically?"

father: "Well son, go ask your mother if she would sleep with Robert Redford for a million dollars. Then you ask your sister if she would sleep with Brad Pitt for a million dollars. Then you ask your brother if he would sleep with Tom Cruise for a million dollars."


So the boy goes up to his mom and asks her if she would sleep with Robert Redford for a million dollars and the mother replies "Oh my god, of course I would, he is so good looking!"
So the boy moves on and asks his sister if she would sleep with Brad Pitt for a million dollars, and she replies "He is so fucking fine, of course I would!"
Then last but no least he goes up to his brother and asks him if he would sleep with Tom Cruise for a million dollars, his brother says "Of course I would, who wouldn't for a million bucks?"

So he goes up to his dad and says "I think I learned the difference between potentially and realistically"
"Well what's the difference?" says the father.

"Well, potentially we're sitting on 3 million dollars, realistically we're living with 2 sluts and a fag!"