A question that’s hard enough to be interesting, but realistic enough that you have some hope of answering it. (Not that you have to find an answer to all beautiful questions; the string theorist Witten, for instance, has never fully answered his biggest questions about the nature of the universe, but he told me that the pursuit of those questions has led him to many other interesting discoveries along the way.)
— A More Beautiful Question: The Power of Inquiry to Spark Breakthrough Ideas by Warren Berger