They Might Be Giants - Letter / Not a Letter lyrics

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.