A man went to his lawyer and told him,
"My neighbour owes me $500 and he won't pay up. What should i do?"
"Do you have any proof he owes you the money?", asked the lawyer.
"Nope," replied the man.
"OK, then write him a letter asking him for the $1000 he owed you," said the lawyer.
"But it's only $500," replied the man.
"Precisely. That's what he will reply and then you'll have your Proof!"