How can you guess that?:

I am an honest person, and am thinking of one of three numbers: 1, 2 or 3. You may ask me EXACTLY one yes-no question, I will answer truthfully, and if you chose the right question,  you will know which number I'm thinking of! (HINT: if I cannot answer your question, I will say I cannot answer it).  (communicated by Rob Andler)