You may know, that sometimes the best answer is to say that the questioner is just asking the wrong question. You may say that in any kind you like (from nice to polite to neutral to offensive to aggressive).
You may be as polite as possible, but sometimes the questioner just asked a "dumb" question (which means: the answer to his question is contained in his question itself).
So, what should you do?
This is kind of re-opening of this question (this was closed, because I was not polite on another website, and yet the original question originates from that other site).