Alan is an honest person who never speaks a lie.
He thinks of a number among 1, 2 and 3.
Now, you can ask him only one question and
that too for which the answer that you will receive,
will be in the form of yes, no or don't know.
But he will reply only truth fully.
What will you ask from his so that you can know the number he is thinking of?
You will ask, I have a number in my mind that i.e. either 2 or 3. Is the number that I have in my mind smaller or equal to the number that you are thinking of?
If Alan replies with no, he is thinking of the number 1.4
If Alan replies with yes, he is thinking of the number 3.
If Alan replies with don't know, he is thinking of the number 2.