"Are you stupid?" — Well, in some cases, I am. In some others, I'm not. But I definitely know that you are. Why? Because you're asking me such a dumb question.
If a person is "dumb" or not, depends on the intellect of the person who's judging. If you ask Elon Musk if Homer Simpson is more dumb than him, he will probably answer you with Yes. But if you ask a person who's more dumb than Homer Simpson (I'm not sure if that's even possible), she will definitely answer you with No (at least if she's being honest). To cut a long story short: The state "dumb" is relative to the judging person's intellect. More clearly: If you're asking a person about being "dumb" in a non-rhetorical context, you're the dumb one.
So now tell me, how should I know by myself if I'm dumb or not? Right, I can't! I would have to compare my own state of intellect with itself. And you should know that comparing things with themselves is technically not possible. But since you've asked me that question, you don't. Therefore I'm free to call you dumb.
If someone asks you if you're dumb, you're free to use the first paragraph as an answer. But beware: If the person who's asking you is smart enough, she will answer with "It's a rhetorical question, you moron."