"Prove It" joke

A quiet little man was brought before a judge. The judge looked down at the man and then at the charges and then down at the little man in amazement. "Can you tell me in your own words what happened?" asked the judge.

"I'm a mathematician dealing in the nature of proof." said the man.

"Yes go on." said the astounded judge.

"Well. I was at the library and I found the books I wanted and went to take them out. They told me my library card had expired and I had to get a new one. So I went to the registration office and got in another line. And filled out my forms for another card. And got back in line for my card."

"Yes go on." said the judge.

"And he asked' Can you prove you`re from New York City?' So I stabbed him."