The song Letter/Not a Letter is performed by They Might Be Giants .
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 that is, it's not a letter
That's a letter
Is that a letter, or what?
That's a letter
Only it's upside down