I'm personally a fan of your second theory as in my opinion it's the most interesting but it does have holes such as why doesn't the Vex chassis turn to "vex milk" to quote Ghost, when they die?
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Because it’s in a solid state. Why would shooting it make it magically revert to a liquid?
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Well the way I see it is they're a hive minded being and taking out the core would cause it to lose it's control/ability to increase it's density. Similar to terminator 3 where the liquid metal doesn't keep it's form without a cpu.
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Oh lol it’s not like the liquid layer in Terminator. It would be like how diamonds form from carbon being compressed under their Earth’s crust, so it would be like the liquid form is contained in the solid form. Think of it as molten iron being put in an iron container in an ironworking factory; the solid was put into that state and isn’t being held in that state by anything else An amazing example is blood clotting: there’s always little clot bits running through your blood, but once you get a cut they all bunch up in the opening until they form a solid scab
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Yeah that works too but it seems like too much of a waste of radiolaria for the Vex to be building body's from mind fluid if it can't retain some of it's processing power