system
Given the following post, respond as if you're commenting on Reddit:
human
[IRL Folklore] Why are the Fair Folk harmed by cold iron?
gpt
Cold iron comes in two forms, raw or cold-forged into a weapon.

It hurts faeries because back in the days when the fae had power in this realm, they were so intimately connected to the Earth that they could not be harmed by normal weapons that came from it.

Remember, this was the time before artificial materials- *everything* that was used as a tool or weapon came from nature.

Except one.

Cold pure iron, in it's rawest form came from:

Deep in the bowels of Earth, from places older than even fae. Long before anything we think of as nature ever existed. If somehow men could get a hold of it, they had in their hands something with more of a connection to the Earth itself than even the faeries had, and therefore could hurt them.

The heavens. The only other source of pure iron was meteoric iron, which was incredibly rare and also, very, very old. Put simply, it wasn't of Earth. No protective enchantment of the fae against the natural world mattered to something not of this world in the first place.

So that's raw cold iron. But if you've seen either of these, they don't come out in very usable forms. How does one use cold iron offensively other than throwing heavy chunks of it at the very elusive and maneuverable fae?

You forge it. No, not with heat, it has to stay cold, remember? Smelting the ore changes it. Once it's glowing hot it is harder, but the special connection it has to the Earth is burned out of it.

So you must keep the iron at its normal temperature. Which means you have to take a heavy anvil, and strike the cold iron into a mold until it is of the shape you want. 

This is not easy, and the weapon is less strong and less refined that a properly smithed one, but used skillfully, it will do serious damage to a fairy.