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The T-1000 is made of a mimetic polyalloy, which is a form of programmable matter (liquid metal, to be precise). In this state the matter acts as both CPU, sensor systems, power systems, etc, which are distributed all across the T-1000's form rather than being localised to a specific area or piece of hardware. This makes it much harder to permanently damage than a standard T-800 combat chassis but at the same time makes it more susceptable to being staggered by lighter gunfire since its likely to absorb all the energy of an impact rather than deflecting it like solid metal would.