Haven't any of you read 'Alice in Wonderland'?
This is a riddle which the Mad Hatter asks. The whole point of it is that it has no answer, although Lewis Carroll later provided this as an answer:
"Because it can produce a few notes, tho they are very flat; and it is never put with the wrong end in front!"
This is a not especially amusing pun on the word 'note', which can be both a short letter (from a writing desk) or a sound (from a raven).