You Can Ask One Question to Find Out if the Number is 1, 2 or 3 – Puzzle

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?

Answer(s) at StackTrace