I don't give smart alec answers to that question, regardless of the attitude of the person asking. I answer seriously. Usually something along the lines of "In case of emergency. Crime happens every day, and the police simply cannot protect you. If a violent crime happened right here, right now, I can tell you as a previous victim who has had to draw a firearm to protect myself the police simply cannot be there in time, no matter how badly they would like to be. So, I carry a gun for the same reason I carry first aid and tool kits in my truck, have a spare tire, have insurance, and have multiple smoke detectors in my home. To take care of myself and my family in the event of an emergency."
Been a couple of times folks have gone from incredulous to "Hmmm...."
"What do you need that gun for?"
1.To stay with current events "that question"
2.Does a waiter ask why you need a fork?
3.So you can continue to use A2 to ask dumb questions
4.Don't worry! there's no water in it.
5.It was written after 1776 when the Germans bombed Pearl Harbor.
It a conversation starter , see it's working already.
All Americans carry guns, may I see your ID?
My tank is in the shop.
I think a better question is "Where is your gun?".
I'll have to agree on the "Zombies" answer as being my most sarcastic. I can usually lead that in to a huge story of apocalyptic proportions. Leaving the asker utterly mystified at whether I think I am serious or just crazy.
My other one is to tell them I need it because I like to fill out a lot of paperwork and wait months and months for replies just for ghits and siggles...