why is a raven like a writing desk?

This is my hat, isn't it cool?
I got it for a costume party last year, and though I didn't technically respect the theme of the party (Disney princesses), it had to be the coolest costume I ever wore - ever.
The other day, I was sitting in my living room and my eyes fell upon my Mad Hatter hat and I asked myself why "In This Style 10/6"?
And so, like any self-respecting iPhone owner, I took to the Internet and found out a couple of things. Apparently there is such a thing called Mad Hatter Day - the 2nd silliest day of the year after April Fool's - celebrated on October 6 in the U.S. and June 10 in the U.K. Get it? 10/6!
This is not where the numbers came from though - quite the opposite in fact, they were picked based on the hat.
Turns out all it is is the hat's tag. The Mad Hatter being a hatter, i.e.: someone who makes hats, had an order to make that hat in that style for 10 shillings and 6 pence. Honestly, now that I know, it seems painfully obvious and I feel like I should known that without looking it up.
Oh well.
Now, if I could just find out why a raven is like a writing desk...