Why is a raven like a writing desk .?

This riddle is famous, and it actually comes from the book Alice in Wonderland, by Lewis Carroll . Many people sit an wonder about the riddle and they don't understand it . Others finds wired reasons like
''Because there is a B in both and an N in neither .'' or like my friend Jakob, yesterday said ''both fly, except the desk'' . And it's all cute, but the answar is right there in front of you .! (or perhaps just for the one who read the book)

Alice is at the tea party with the March Hare, the Mad Hatter, and the Dormouse, when out of the suddenly the Mad Hatter poops the quistion ''Why is a raven like a writing desk .?''

"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.
"Nor I," said the March Hare.
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."

The answer is that there isn't any answer .