So she asks him why is he staring and he answers, "I have a question I need to ask you but I don't want to offend you."
The nun replies, "My dear son, you cannot offend me. When you're as old as I am and have been a nun as long as I have, you have had a chance to see and hear just about everything. I'm sure that there's nothing you could say or ask that I would find offensive."
The cab driver hesitates for a moment and then says, "Well it's like this; I've always had a fantasy to have a nun perform oral sex on me."
The nun replies, "Ok well, let's see what we can do about that, shall we. There are two conditions though, firstly you have to be single and secondly you must be Catholic."
The cab driver is very excited and says, "Yes, yes! I am single and I'm Catholic too!"
The nun then says, "Ok then, pull into the next alley."
The cab driver does so and the nun duly goes ahead and fulfills his fantasy. They get back on the road and start driving again, but the cab driver soon starts to cry.
The nun sees this and asks him, "My dear child, pray tell, why are you crying?"
The cab driver says, "You must forgive me sister, but I have sinned. I lied to you, I must confess that I'm married and I'm also Jewish."
The nun laughs and says, "That's OK, my name is Kevin and I'm on my way to a Halloween party."