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It depends. Sometimes I don't mind answering. If it's something that I don't want to answer, I'll tell the students that its a "closely guarded secret" and "curiousity killed the cat" etc.. Or jokingly tell them if you ask this again, you'll lose marks!
Basically try humour to move them away from the topic or if that doesnt seem to be working, I tell them they can stay after school and I'll tell them (which never happens :)
I agree with MR Hasseeb Anjum