Medical books, since childhood. Hey, we've all got a body, why not know how it all works? I love to read them just to learn. I've been doing this for over 20 years. The internet has vastly expanded my options for learning as well, it's all just a click away! I have to ration time for this hobby, or I'd become immersed.
Also, exercise. Mostly because of the first habit, and knowing the innumerable benefits of doing so on a regular basis.
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They tell me I think too much. I tell them they don't think enough.