His Dad thinks for a while and then says "Right son, go and ask your mother if she'd sleep with Brad Pitt for a million quid."
The boy trots off and comes back saying "Dad, dad, she said she would! She would sleep with Brad Pitt for a million pounds."
"OK son," says his dad. "Now go and ask your sister the same question."
The boy toddles off, and comes back saying "Dad, dad, she said she would too!"
So then his dad says "Right, son, now go and ask your elder brother if he'd sleep with Brad Pitt for a million pounds."
The son comes back excitedly saying "Dad! Dad! He said he would too!"
"Well there you have it, son," said his dad. "Theoretically we could be sitting on three million quid - Realistically we're living with two tarts and a poof."