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I usually ask "What are you interested in?" and just let them speakI've tried that a few times but never really got any decent results out of it. (Disclosure: I conduct ~100 interviews a year.)
The question is too open-ended, so IMO it's akin to asking a child "what did you do at school today?". The answers for that are just non-specific and often the candidate falls back to describing what they currently do at their work. Hell, if they are interviewing for a new job, they are looking for a CHANGE, and I learned just one thing they probably don't like. Instead in preparation I go over the candidate's CV and try to gauge what kinds of technologies, environments and problems they have been exposed to. And then I pick the most interesting and/or longest ones as items of discussion.
Most of the times I find that the candidates are happy to tell about their experiences and voice their opinions. This has the dual benefit of allowing the candidates to better relax, and gives me a glimpse into what kinds of details catch their eye. The very best experiences are those where the candidate forgets they are in an interview and end up treating me to a wonderful lecture on a topic that I have not known enough about. When that happens, I always feel cherished.