One question to know if the number is 1, 2 or 3 I've recently heard a riddle, which looks quite simple, but I can't solve it.

A girl thinks of a number which is 1, 2, or 3, and a boy then gets to ask just one question about the number. The girl can only answer "Yes", "No", or "I don't know," and after the girl answers it, he knows what the number is. What is the question?

Note that the girl is professional in maths and knows EVERYTHING about these three numbers.

EDIT: The person who told me this just said the correct answer is:

 "I'm also thinking of a number. It's either 1 or 2. Is my number less than yours?"


Let the boy ask the girl "Subtract 2 from the number your are thinking and then take the square root of the result.Is the result positive?".
If the girl replies "yes" then the number is 3 because square root of 3-2=1 which is positive.
If the girl replies "no" then the number is 2 because square root of 2-2=0 which is not positive.If the girl replies "I don't know " then the number is 1 because square root of 1-2= is imaginary number which cannot be said whether it is positive or negative.
                                   OR                                                                                          
Let the boy ask the girl"Divide the number you have with the previous number.Ask if the result is a fraction?"If the girl replies "yes" the number is 3 because 3/2 is a fraction.If the girl replies "no" then the number is 2 because 2/2 is not a fraction.If the girl replies "I don't know" then the number is 1 because 1/0 is undefined. 
