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?"