P versus NP problem
A computer may be able to tell if an answer is right, but take longer to get the answer. For some interesting, practical questions of this kind, difficult answers are possible to check quickly. So NP problems may be thought of as being like riddles: it may be hard to come up with an answer to a riddle, but once one hears the answer, the answer seems obvious. In this comparison (analogy), the basic question is: are riddles really as hard as we think they are, or are we missing something? Is there a secret to always having an answer?