Become a fan of h2g2
In Chapter 7 of Alice In Wonderland, 'A Mad Tea-Party', the Mad Hatter puts this riddle to Alice. Unfortunately, neither he nor Alice know the answer.
Here are three of the most brilliant attempts:
James Michie - because each begins with an e.
Aldous Huxley - because there's a b in both, because there's an n in neither.1
Not forgetting, "Because they both can fly, except the desk," from Paul H, an h2g2 researcher.
Peter Heath, in The Philosopher's Alice raises the point that the riddle has an infinite, or finitely huge, number of answers, i.e. all the negative properties that a raven would share with a writing-desk. For instance, neither is the Eiffel Tower, each is a not-rabbit, each is not-pink, presumably, and so on. 2
Contrarywise, it is also a WHY question, asking for the reason for any resemblance. The answer to WHY a raven is like a writing-desk, in this sense, could be 'Just because it is', or 'God made it that way', or 'That's the way things are', and so forth. WHY questions, like these, are notoriously difficult to answer satisfactorily, or may even be unanswerable outside of a religious or ideological context. Ask any parent!