dumb question?

If there is no such thing as a “dumb question,” why is there apparently such thing as a “great question” or an “an interesting question”? Over and over, I hear really smart people say things like “that is a great question”, and I think this must mean that the speaker is judging a question on a scale of quality. My question is thus do “dumb questions” really exist, and can they be identified as that question which is not followed by “that’s a great question”?