Conundrum

Part of speech: noun (n.)

1. A kind of riddle based upon some fanciful or fantastic resemblance between things quite unlike; a puzzling question, of which the answer is or involves a pun. Or pun ambiguous, or conundrum quaint. J. Philips.

2. A question to which only a conjectural answer can be made. Do you think life is long enough to let me speculate on conundrums like that W. Black.