"Meaning of... 'potentially' and 'realistically'" joke

Meaning of...' potentially' and' realistically'
A young boy went up to his father and asked him, "Dad, what is the
difference between' potentially' and' realistically'?"
The father thought for a moment, then answered, "Go ask your mother
if she would sleep with Brad Pitt for a million dollars. Then ask
your sister if she would sleep with Brad Pitt for a million dollars,
and then, ask your brother if he'd sleep with Brad Pitt for a million
dollars. Come back and tell me what you learn from that."
So the boy went to his mother and asked, "Would you sleep with Brad
Pitt for a million dollars?"
The mother replied, "Of course, I would! We could really use that
money to fix up the house and send you kids to a great university!"
The boy then went to his sister and asked, "Would you sleep with Brad
Pitt for a million dollars?"
The girl replied, "Oh, good heavens! I LOVE Brad Pitt and I would
sleep with him in a heartbeat. Are you nuts?"
The boy then went to his brother and asked, "Would you sleep with
Brad Pitt for a million dollars?"
"Of course," the brother replied. "Do you know how much a million
bucks would buy?"
The boy pondered the answers for a few days and then went back to his dad.
His father asked him, "Did you find out the difference between
' potentially' and' realistically'?"
The boy replied, "Yes,' Potentially', you and I are sitting on three
million dollars, but' realistically', we're living with two hookers
and a future congressman."