Mine changes a lot, mostly because I'll get really into something but then something happens that breaks me off from doing any more research on it, like a long-term loss of internet. I think my biggest on and off obsession is languages. I can write in Dwarvish (from the Hobbit) quickly and efficiently (no one could read my diary if they tried!), and if I am able to refer to my journal I can also write in Quenyan characters (Elvish language from LOTR, can't yet write in actual quenyan. Simply english in Quenyan characters) Interestingly enough I've never even been that big of a fan of LOTR and the Hobbit, I just really enjoy the languages. For a bit I knew how to write some phrases in Ethiopian as well, I learned from a friend I met at a mental hospital. (Suicidal thoughts... long story for another time) I keep a journal in which I put a guide to writing in several alternative alphabets and codes, including Quenyan, Dwarvish, Morse, Sky Hylian (Zelda), Matoran (LEGO), Alien (Futurama), and Gnommish (Artemis Fowl). I actually have quite a reputation at my (rather large) school for being able to write in Dwarvish; people (particularly boys) stop me all the time to ask about it. (And shortly after ask for hugs. It's beginning to frighten me.) And I think I'm the only kid in my Spanish class that actually enjoys the class and is enthusiastic about it.
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Come along, John. And bring the salt.