There are no difficult interview questions... though if you are all tense and scared, probably every question is a difficult one. I have had a healthy amount of interviews in the last few months as I am contemplating my next steps, and I honestly don't recall any questions I would call "hard". Any interviewer worth their salt wants to understand how you think, what kind of person you are, and whether you know what you are talking about rather than get an answer to a specific question. When I ask questions myself on the other side of the table, I am not looking for any predetermined answer either - an interview is not a math class, life has very few definitive right and wrong answers. So relax, be honest and be yourself - and you will do better than fretting or giving carefully rehearsed responses.