Actually, "Munchkin Bites", "Super Munchkin", "Munchkin Fu", "Star Munchkin", "Munchkin Booty", "The Good, the Bad, and the Munchkin", "Munchkin Cthulhu", "Munchkin Impossible", and the new "Munchkin Axe Cop" aren't expansions - they're standalone games, built along the same lines as the original "Munchkin", but with their own genre assumptions (the original uses D&D-style hack'n'slash genre, "Bites" is vampire-style, GB&M is Westerns, and so forth). Most of them have at least one expansion of their own, with additional cards.
Even worse, it's possible to mix-and-match the sets (there are included rules for doing so), so it's possible to eventually wind up with a Dark Half-Orc Half-Pirate Master Wizard with a Flaming Gleaming Narrow S-Cape of Doom!.
The original set has multiple expansions - they're getting set to release the eighth expansion, introducing centaurs, called "Half Horse, Will Travel" (yes, in order to enjoy this game, it is necessary to enjoy puns and absurdist humor) - but each expansion requires the core game to play, and cannot be played alone. This may seem a bit nitpicky, but if you go into a gaming store and ask for a Munchkin expansion, they'll quite happily send you home with, say, "Munchkin 4: De-Ranged", which you won't be able to play because you don't have the core Munchkin set. Oh, sure, you can giggle at some of the monsters and enhancement cards, but that's only fun for so long...
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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.