I'm thinking of a number. The number I'm thinking of is either 10, 20, or 30.
You may ask me one question to which I can either reply, "yes", "no", or, "I don't know".
What one question do you ask me so that you can determine what number I'm thinking of ????
Don't bother asking, "what number are you thinking of", or anything like that, 'cos I can only reply, "yes", "no", or, "I don't know".