Short answer: It's not really an appropriate question for an interview situation.
I've been a programmer for 20 years and I couldn't answer this off the cuff in an interview. I would find it one of those "WTF?" type questions that doesn't make any sense to ask. It seems to me to be more of one of those over-a-coffee-in-the-lunchroom type conversations you may have with a co-worker.
I'm struggling to see what value there is. What are you trying to determine with it? How will identifying a specific piece of code inherently improve your perception of the candidate? If you want this, ask the poor candidate to bring something with them to the interview, or beforehand.
I've been through many interviews as both interviewer and candidate. If you want to ask them what they like or find interesting, ask them that. You might find you will gain more insight than an ambush-type question :)