This article is from the Books FAQ, by Evelyn C. Leeper firstname.lastname@example.org with numerous contributions by others.
According to Martin Gardner, Carroll had no answer in mind which he first
wrote this. However, Carroll did gave a solution himself, in an 1896
edition of "Alice": "Because it can produce very few notes, tho they are
very flat; and it is nevar [sic] put with the wrong end in front." Gardner
has recently added another: "Because there is a 'b' in 'both.'"
In a brief preface that Carroll wrote for an 1896 edition
of ALICE IN WONDERLAND, he said he had no answer in mind
when he gave this riddle. Many answers have since been
suggested, including one by Carroll himself, some of which
you will find in my AA note. In 1989 England's Lewis
Carroll Society announced a contest for new answers, to be
published eventually in the society's newsletter,
Aldous Huxley, writing on "Ravens and Writing Desks"
(Vanity Fair, September 1928), supplies two nonsense
answers: because there's a 'b' in both, and because there's
an 'n' in neither. James Michie sent a similar answer:
because each begins with 'e'. Huxley defends the view that
such metaphysical questions as: Does God exist? Do we have
free will? Why is there suffering? are as meaningless as
the Mad Hatter's question -- "nonsensical riddles,
questions not about reality but about words."
"Both have quills dipped in ink" was suggested by reader
David B. Jodrey, Jr. Cyril Pearson, in his undated
TWENTIETH CENTURY STANDARD PUZZLE BOOK, suggests, "Because
it slopes with a flap."
Denis Crutch ("Jabberwocky," Winter 1976) reported an
astonishing discovery. In the 1896 edition of ALICE,
Carroll wrote a new preface in which he gave what he
considered the best answer to the riddle: "Because it can
produce a few notes, tho they are *very* flat; and it is
nevar put with the wrong end in front." Note the spelling
of "never" as "nevar." Carroll clearly intended to spell
"raven" backwards. The word was corrected to "never" in
all later printings, perhaps by an editor who fancied he
had caught a printer's error. Because Carroll died soon
after this "correction" destroyed the ingenuity of his
answer, the original spelling was never restored. Whether
Carroll was aware of the damage done to his clever answer
is not known.
Another answer is that Poe wrote on both.
Dan'l Danehy-Oakes also suggested the variant that both have inky quills.
In chapter 39 of THE SHINING, Stephen King says,
"The higher the fewer, of course! Have another cup of tea!"
The latest answer is from Martin Graham (B7337@GTE.NET):
In a LEWIS CARROLL--FRAGMENTS OF A LOOKING-GLASS by Jean Gattengno and
in the first biography on Carroll by his nephew Stuart Dodgson
Collingwood, THE LIFE AND LETTERS OF LEWIS CARROLL, we learn of a few
facts regarding Carroll's (Dodgson's) intrest in the occult. We learn
that "Mr. Dodgson took a great interest in occult phenomena, and was
for time an enthusiastic member of the 'Psychical Society.'" Also we
learn that Carroll had a specific interest in automatic writing. We
also learn when consulting any good dictionary of Symbols that Ravens
are believed to be messengers between the land of living and the land
of the dead. Automatic writing is also used to communicate with the
dead. Thus, though the answer to the riddle taking these factors into
account is not especially humorous, it seems that the correct answer to
this riddle should be.... A raven is like a writing desk because one
might communicate with the dead through either.