“Why is a raven like a writing-desk?” - the Mad Hatter in Alice in Wonderland.
Although, according to Carroll himself, the riddle originally had no answer, he supplied an “afterthought” answer in his preface in a later edition. His answer was that it can produce a few notes, though they are very flat, and it is never put with the wrong end in front. Of course, this opened up a plethora of unique, proposed answers and contests.
Here are my favorites:
1) Both have quills dipped in ink – David B. Jodrey
2) Because one has flapping fits and the other fitting flaps – Peter Veale
3) Because a writing-desk is a rest for pens and a raven is a pest for wrens - Tony Weston4) Because it bodes ill for owed bills - Francis Huxley
(answers taken from The Annotated Alice: The Definitive Edition by Martin Gardner)