Five
Apparently, Jimmy Wales (founder of Wikipedia) says that a wiki / blog needs five committed contributors to be self-sustaining. I posted to the Global Guerrillas blog entry instead of the source… because the original post is looooooooooooooong.
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I remember reading that it takes at least six molecules to demonstrate the behaviour of all three (common) phases of matter, but can’t find the source. I’ll document that, when I get a reply.
(The basic premise is that, two molecules is enough to get solid or gas behaviour — it just depends whether the molecules are bonded or not — but liquid behaviour involves a more complex interaction of molecules, and thus needs more than just a couple molecules to “define”.)