why is a raven like a writing desk?

this splendid rhetoric comes from lewis carroll's alice's adventures in wonderland

"Have you guessed the riddle yet?" the Hatter said, turning to Alice again.
"No, I give it up," Alice replied. "What's the answer?"
"I haven't the slightest idea," said the Hatter.
Alice sighed wearily. "I think you might do something better with the time," she said, "than wasting it in asking riddles that have no answers."

this is a riddle that has gone unanswered for generations. it has no real answer, really. perhaps old carroll was too half-witted to come up with one. or mad, even.