I always was the kid who'd read encyclopedias for fun, so whenever I came upon a topic in my writing that required research, I dug in with relish. Topics I've had to research for my writing included various forms of swordsmanship, mixed martial arts, wildlife biology, various kinds of mythology and spirituality from all over the world, the quirks of various types of firearms, lots and lots of medical and emergency response stuff (as part of my day jobs), and conspiracy theories along with how to disprove them. (I don't recommend going too far down that rabbit hole, as some of the people I've encountered when researching some conspiracy theories about, say, 9-11 or the Apollo missions are downright scary and have done some horrible things to my husband and me.)
Lately, though, it's mostly military and aviation history, and it's gotten to the point that some of the people assisting with my research think I should actually learn how to fly something other than simulators and drones.