Chemistry set, microscope set...finding out you could buy oxidizers at the drug store even though you were only 10 years old and were reading a LOT of science fiction, which led to reading the Model Rocketry Handbook, which led to "how to make your own rocket fuel using the oxidizers you could buy at the drug store"....

And of course that led to astronomy and how stars worked and astrophysics and nuclear physics and before I was out of high school I'd figured the critical mass for an atomic weapon AND how to make it work, if only I had some Pu239 or U235 and could talk my brothers into doing a little file work....