In my interview, what if they ask me a question on a topic I know nothing about?

It’s tempting, in this situation, to panic and rush into the subject, but this usually results in a garbled splurge of word vomit, and what they want to see is that you think in a calm and logical way. So try and pause, breathe in and out, or count to three, and then ask if you could have a minute to think about it. They have no problem at all with you asking that, and pausing (also I promise the silence will seem so much longer and louder to you than them). It’s far better to have a few quiet seconds followed by a thoughtful and insightful response, than that dreaded word vomit. When you’ve had a chance to gather your thoughts into a coherent shape, take them through your thinking. It doesn’t matter if you don’t ‘know the answer’ (there usually is no right answer), what they really want to see is HOW YOU THINK. One good tactic is to say that you aren’t a hundred percent sure of the reference, for example, that they’re referring to, but using the insight of ‘x’ (something/someone you ARE confident in discussing) you think that ‘y’. This shows that you’re (a) aware of how much you DON’T know (a very attractive quality in an Oxbridge tutee in the eyes of a supervisor) and (b) are flexible in your thinking. If they argue with you and say that you’re wrong, don’t feel you have to cling to your idea for the sake of it and be stubborn - its good to show that you’re flexible in your thinking and willing to learn, and adapt to their idea. If you DO disagree with the interviewer, and are able to defend your idea articulately, then do so. Try to show, however, that you are also taking their ideas into account and working with them, and do not be dismissive. It’s totally fine to ask questions as well - in fact, it’s great - you want it to feel more like a scintillating conversation than an interview.