That's a strange reaction.. Are you always able to answer any quesion that comes up at work? I know I don't. And so seeing people's reaction to qestion that is

That's a strange reaction.. Are you always able to answer any quesion that comes up at work? I know I don't. And so seeing people's reaction to qestion that is too hard for them is a valuable signal. As long as one tells candidate in advance: "I am going to be asking a mix of questions, some easy some hard. I don't expect you to be able to answer every question", this sounds like a good question.