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This question already has an answer here:

These aliens have bodies that have evolved to a degree where they can effortlessly handle various extreme environmental conditions, they are super-strong and super-resilient to blunt force trauma and bullets, yet a knife or a sword or some other metal blade cuts through them like butter.

What could make this charasteristic feasible?

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marked as duplicate by kingledion, Andon, Logan R. Kearsley, L.Dutch, JBH Jan 20 '18 at 4:31

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  • $\begingroup$ When I read the title I assumed you meant freeze rays. I've never heard swords and knives referred to as "cold weapons" before. $\endgroup$ – F1Krazy Jan 19 '18 at 17:21
  • $\begingroup$ @F1Krazy I think I read it somewhere but still changed the question so there won't be any confusion. Thank you! $\endgroup$ – JanT Jan 19 '18 at 17:28
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    $\begingroup$ I'm pretty sure I've seen a question much like this, about how creatures can be vulnerable to stabbing but not bullets, or maybe the other way around, but I can't find it at a glance. Anyway, this doesn't appear to be about the perception of science within an imaginary world, so I removed the [science] tag. $\endgroup$ – a CVn Jan 19 '18 at 18:05
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    $\begingroup$ @F1Krazy Although somewhat archaic, "cold iron" and "cold steel" used to refer to forged items after they went through the cooling process and was often used in conjunction with weapons. You see it a lot in fantasy where it is sometimes applied to some specific and special forging process or some exotic type of iron, but historically it appeared frequently just to mean forged. $\endgroup$ – TimothyAWiseman Jan 19 '18 at 18:59
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    $\begingroup$ @MichaelKjörling This one?worldbuilding.stackexchange.com/questions/13024/… $\endgroup$ – jkeuhlen Jan 19 '18 at 20:11
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Perhaps the creatures skin/musculature is woven (think fabric) and excels at spreading out impacts over a larger area, similar to a Kevlar vest? Just like Kevlar it wouldn't be that great against blades and the like, since the blade would just be cutting individual fibers of their skin or muscle as it travels along.

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    $\begingroup$ Kevlar is good against swords but a fiasco agains needles (unles driping in a non newtonian liquid) $\endgroup$ – jean Jan 19 '18 at 23:13
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Subcutaneous dilatant tissue

Dilatant (or shear-thickening) material is a nifty kind of non-Newtonian fluid that hardens proportionally when shear stress is applied to it. Body armor has been developed on this basis. When you get shot by a bullet, the liquid instantly toughens when it's struck, deflecting the blow. If your creatures had this kind of tissue under (or instead of) their skin, they would deflect projectile weapons including bullets, and blunt weapons, with ease.

A sword-wielding warrior, on the other hand, could attack with a slower, forceful push to part the material without triggering its dilatant properties, and reach the squishy guts inside with the tip of the blade. A katana technique (where the sword is dragged parallel to the enemy) might also work.

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    $\begingroup$ In the pure science fiction world, Dune's shield generators had similar properties. Indeed, they were fabricated by Frank Herbert with the expressed intent of making guns less important, encouraging knife and sword fighting instead. $\endgroup$ – Cort Ammon Jan 19 '18 at 21:51
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Kevlar-type fabrics are very resistant to blunt force trauma, such as from bullets, but if you have a sharp-pointed knife you can stab someone to death through a Kevlar vest because the fabric weave isn't tight enough to stop the knife from penetrating, and then the rest of the knife shoves through / cuts the weave as it goes through. Meanwhile, if a bullet with a relatively blunt nose hits the vest, it disperses the energy across the surface of the vest like catching a ball in a net. Bottom line: If your critter has a skin designed like a bullet-resistant vest, your critter can be resistant to blunt force trauma (like from a bullet) while still being easily killed with a common Ka-Bar.

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Preventing Regeneration

In Jim Butcher's Furies series most of the nobility is able to regenerate pretty quickly, but if the wound is cauterized, they can't. So they use blades so hot that they cauterize the wound and prevent healing.

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  • $\begingroup$ OP's creatures are adapted to extreme environmental conditions, which would probably include heat. $\endgroup$ – SPavel Jan 19 '18 at 23:15
  • $\begingroup$ @SPavel heat is not the not the same as burning. Even then a burn does not guarantee cauterization and would only be a surface burn that might take longer to heal (days or weeks), and not an internal cut unable to heal on the spot and therefore allowing them to be killed in combat. The only problem is that guns could be made to cauterize by heating the metal and using a different firing mechanism. So this answer ironically allows bullets to work. $\endgroup$ – The Great Duck Jan 19 '18 at 23:23
  • $\begingroup$ Superheated bullets fired by railgun? I could see those causing cauterized wounds. $\endgroup$ – SPavel Jan 19 '18 at 23:25
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    $\begingroup$ @TheGreatDuck Having cauterized sword slashes through your midsection (severing necessary organs and such) would be very different from a cauterized bullet-hole. Un-cauterized bits of flesh could likely grow back in place of the cauterized part for the bullet hole, whereas getting chopped in half is more ... permanent. $\endgroup$ – Blapor Jan 19 '18 at 23:45
  • $\begingroup$ @Blapor true but it could at least be a weapon of war. It would still work given enough bullets and enough firing power. Certainly automatic machine guns would have the firepower? $\endgroup$ – The Great Duck Jan 20 '18 at 3:56

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