Thinking on this topic, I was wondering how to categorize questions which may not necessarily be "stupid," but more in the realm of annoying and puerile.
I'm never annoyed by someone asking an honest question about something they don't know, even if it's something that appears obvious or something that everyone above a certain age should know. But there's no point in getting annoyed or irritated by it. The answer may be useful to somebody.
The kind of questions I find annoying and sometimes rude are those of an intrusive or personal nature, when the answer has absolutely no bearing on anything, nor can be considered a good faith effort to gain information. Cops do that because they can, but when other people do it, that's where I draw the line.
I'm not even that fond of being asked questions like "What is your favorite color?" I really don't have one.