I have one of the three numbers: 1, 2 or 3 in my mind. I speak only truth. You can ask me just one question for which I will only reply in yes or no or don't know. What question will you ask from me so that you are able to know the number?
You will ask, I am thinking of a number from 2 and 3. Is the number I am thinking of smaller than or equal to the number you have in your mind?
Now, if I reply no, then the number is 1.
If I reply yes, then the number is 3.
If I reply don't know, then the number is 2.