Why Is A Raven Like A Writing Desk?

I just watched the movie Alice in wonderland, and I couldn't help but wonder, Why is a raven like a writing desk? This is something the mad hatter asks Alice quite a bit, and the answer to this quirky riddle was never given :( I tried googling it, but no luck! now I cannot get it out of my mind! This is going to bug me until I figure it out. So I ask of you all a favor, get your best idea of why a raven is like a writing desk, and leave it in a comment below :) Much appreciated!