EDIT: An important correction was made to this post near the very end. Please scroll down for the full scoop.
Since I was a kid though, I have had an immediate answer for this supposedly profound question. (I say "supposedly profound", not because I'm belittling Zen Buddhism, but because I've always been way too literal-minded. The question never struck me as being 'profound' in any sense.) My answer when asked that question? "Pointless."
At first blush, my answer may seem thought-provoking; as if someone had asked me, "How much is 7 minus 3?" and I answered, "Orange." This casual listener, perhaps slightly familiar with Zen Buddhism, might suppose that I was seriously attempting to probe the deeper-meanings of myself, my being, and how I relate to my universal "one-ness". They could be excused for thinking that I was attempting to give a 'profound, meaningful, inner answer' to the 'profound question'.
I hate to disappoint anybody, but here's what actually goes on in my head: