The most difficult questions I received pertained to whether I understood the work which I was describing. So, I talked about some of the research which I worked with in my undergraduate career - My interviewer asked some questions regarding whether these aspects were comerciallizable, how they might progress into the future, etc. I don't remember any of the particular questions because they didn't come at my with the crazy riddles that everyone is afraid of. The questions were straight forward, but pointed, and I had the sense that the "feel" of my delivery and confidence were as important as the answers themselves.
