A lot of the Resistance guys have tattoos, actually.
John doesn’t—his face is an identifying mark enough, for both sides. Skynet doesn’t need more clues to know who he is, and no human soldier worth his salt could forget. But the rest of them do.
It’s a way to keep busy during long watches, and a way for even the youngest recruits to prove their toughness,
by standing up to the pain and the risk for infection with a still face and without screaming.
They’ve mostly mastered the sterilization issue by the time Kyle comes up through the ranks, though numbing agents are still hard to come by.
It’s also a way to prove their humanity in the basest sense: ink shows up wrong, somehow, on synthetic skin.
So if metal’s really working you, trying to infiltrate without violence, it’s a quick tell—and it’s something Skynet hasn’t figured out yet, either,
well after the machines have mastered bleeding, birthmarks, and scarification.
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