Creationists: "If God didn't create the universe and life, then who did?"
Atheist: "Besides the fact that you asking "who" implies that there is a "who" to inquire upon, let's say that I don't know. I could look up scientific evidence for how the universe and humans came about through natural causes, but for the sake of the argument, I'll say I don't know. So what? Me not knowing doesn't make you correct. You still have to prove your claim, which you so far haven't."