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."