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 demonstrate. 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 decides to ask his mother. "Mom, if
someone gave you a million dollars, would you sleep with Robert Redford?"
"Don't tell your father, but yes, I would.

"
He then goes to his sister's room. "Sis, if someone gave you a million
dollars, would you sleep with Brad Pitt?"
She replies, "Omigod! Definitely!"
The kid goes back to his father. "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."