Mathematics, Physics, Philosophy, Machine Learning, Existential Risk, Politics
That the interviewers aren't trying to catch you out. They're just wanting to examine how you learn and whether the tutorial/supervision style of learning will work for you.
I enjoy debating almost any topic and I train and participate in triathlons.
I have significance experience tutoring over 75 students of varying age (11-19) and ability including Oxbridge applicants, A Level and GCSE students, and preparing 11+, 13+ and 16+ candidates.