http://www.straightdope.com/classics/a5_266.html
"This riddle is very famous, although it is the rarefied kind of fame that entails most people never having heard of it. It comes from Lewis Carroll's Alice in Wonderland. Alice is at the tea party with the March Hare, the Mad Hatter, and the Dormouse, when apropos of pretty much nothing the Hatter pops the question above. Several pages of tomfoolery ensue, and then:
"Have you guessed the riddle yet?" the Hatter said, turning to Alice again.
"No, I give it up," Alice replied. "What's the answer?"
"I haven't the slightest idea," said the Hatter.
"Nor I," said the March Hare.
Alice sighed wearily. "I think you might do something better with the time," she said, "than wasting it in asking riddles that have no answers."
At this point most of us are thinking, ho-ho, that Lewis Carroll, is he hilarious or what? But inevitably you get a few losers who say, well, OK, but I still want to know why a raven is like a writing desk. One sighs wearily. Guys! It's a joke! The answer is that there isn't any answer!"
Carroll got so many inquiries he decided to make an answer, feeble as it was:
"Lewis Carroll had this riddle riposted at him so many times that he wrote the following in his preface to the 1896 edition of his book:
Enquiries have been so often addressed to me, as to whether any answer to the Hatter's Riddle can be imagined, that I may as well put on record here what seems to me to be a fairly appropriate answer, viz: "Because it can produce a few notes, tho they are very flat; and it is nevar put with the wrong end in front!" This, however, is merely an afterthought; the Riddle, as originally invented, had no answer at all."