Rhetorical statements
How do you make rhetorical statements?
The context is, your friend asks you something and you realize you don't know it. The friend asks you why you don't know, you'll check it out later but now you should say something to the friend.
1. I don't know what I don't know.
2. I say I don't know something that I don't know.
3. What's wrong with saying I don't know something that I don't know?
Which one makes sense?