Letter / Not A Letter

What's that?
That is a letter.
What is that?
That's not a letter.
That's a letter.
That's not a letter.
That is a letter.
Is that a letter?
I don't know what that is.
That's so a letter.
What is that?
Whatever it is, that's not a letter.
That's a letter.
Is that a letter, or what?
That's a letter. Only it's upside down.