According to the 1865 novel, Alice's Adventures in Wonderland, the Mad Hatter asks, "Why is a raven like a writing desk?" Because there was no response, Alice was unable to solve the riddle. The Mad Hatter was the first to admit it. This riddle is well-known, albeit it's the kind of well-known that most people have never heard of.
"Have you find out the answer of the riddle?" the Hatter inquired, turning back to Alice.
"No, I'm giving up," Alice said. "What is the solution?"
"I haven't the slightest understanding," the Hatter said.
"Neither do I," the March Hare replied.
Alice sighed, exhausted. "I suppose you should be doing something more productive with your time than asking riddles with no answers," she suggested.
The puzzle went on to become one of the most renowned riddles in the world. Carroll did not intend for there to be an answer, but in a later book marketing services edition, she did provide an answer to the puzzle.
"Because it can generate a few notes, though they are quite flat, and it is never set with the wrong end in front!" Carroll wrote. According to Gizmodo, Carroll also misspelled never as "nevar," which is raven spelled backward, but a proofreader removed the smart joke.