Why is a raven like a writing desk?

Why IS a raven like a writing desk? I've been wondering ever since this  piece of wall art, purchased in 2012 -

has sat on my shelf. (The clock is from The Cardboard Clock Company, round about 1991 - and the vase, much used in tulip season, is from the Chelsea Craft Fair a few years before that, maker Maggie Byrne.)

The paper has yellowed in the daylight over the years, but the question remains.

Coincidence has now revealed why a raven MIGHT BE like a writing desk. Looking for something else, I found "top ten intriguing riddles from history", among them this one, set by Lewis Carroll in Alice’s Adventures in Wonderland.

"Alice is at a tea party with the Mad Hatter, who decides to give her a riddle: “Why is a raven like a writing desk?” No answer is given because, when Alice gives up, the Mad Hatter admits he doesn’t know the answer either. Many famous people have subsequently put forth their own solutions, with the best probably being author Aldous Huxley’s “Because there is a B in both and an N in neither,” and puzzle enthusiast Sam Lloyd’s “The notes for which they are noted are not noted for being musical notes.”

However, Carroll himself actually did provide an answer to his famous riddle, mostly because fans would not stop hounding him about it. 

Carroll’s Answer: Because it can produce a few notes, though they are very flat; and it is nevar put with the wrong end in front! (Note the spelling of “nevar.”)