I used to get caught up in others' fictional universes, most notably Star Trek, Larry Niven's Known Space, and James White's Sector General stories. Then, in college, I was introduced to Role-Playing Games...
Aathe is the world for my D&D games. I've constructed its history, both legend and the events underlying the legend (for instance, the war that tore the old world apart twenty thousand years ago and introduced magic into things also left the people with a belief in, and fear of, the Tenth God, Technos the Uncaring, whose machines destroy reality).
I've also built a history for my Champions superhero universe, starting with the first costumed adventurer from the 1920s, the Black Cat ("he's bad luck for the bad guys"). Today it includes the UN's Blue Star Squadron (which originated as an Allied multinational force in WWII, against the Nazi Ubermenschen), the Russian Red Star Squadron, the Chinese People's Heroes, the international independent Terran Defense Force, and Black Ops, the secretive enforcement arm of the US government's Bureau of Transhuman Affairs, as well as a number of independent groups and individuals...
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.