"Why is a raven like a writing desk?" I love hard riddles. Even if a riddle was designed to have no solution, we can often think of good answers anyway (e.g. Poe wrote on both).
"Riddles: Sherlockian, Joycian, Freudian "eye","
Word Ways: Vol. 44
, Article 11.
Available at: http://digitalcommons.butler.edu/wordways/vol44/iss4/11