It was also foreshadowed in the Drill episodes when Aang said, "What I'd give to be a metal bender."
Hatneas
Metalbending makes sense because they establish Toph isn't folding the metal directly. She's using earth bending as leverage.
Its the classic trope of creatively using your powers to pull off different feats
mullerpotgieter
It also helps that the audience is learning about metalbending along with Toph.
SillinessIsCartoons
The Lion Turtle was a deus ex-machina. This felt more like a natural extension of earthbending, like how water had ice and eventually blood, or how fire had lightning.
YungandRekless
Toph dropping into that metal door to bend a suit of armor around herself is an awakening moment. I love how brutal earthbender movements are.
Takyomi
It's not just that she had Seismic Sense, but also that she was trapped in a metal cage and banging her hands and feet against the cage in frustration. Doing that enough times gave her the opportunity to sense the earth in the metal. They show the impurities emerging in her vision after she hits the metal, which is what gave her the idea to try bending it.
irohdw
This also explains why fire benders can use lightning. Fire is essentially energy as zuko found out with the dragons but most see it as flames that destory especially masters
Converting energy into electrical power with build up can make sense
ivanbluecool
It was the most brilliant thing. The gurus knew that metal was a refined earth, but only Toph could actually see it to work with it.
robertbeisert
It helps that this happens early enough for the show to fully explore its strengths and limitations. Toph cannot do everything with metal that she can with earth; its greatest advantage is simply that nobody else believes it's even possible.
YOSHIERIDER
Another important thing is metal isn’t a very commonly used or accessible material. The fire nation went full industrial revolution with factories, ships, weapons, etc, but most of the world doesn’t seem to use metal much outside objects like swords, jewelry or small components like nails. Most things are made out of stone or wood or any other natural materials are around.
jackventure
I binged this show for the first time a few years ago. The whole of all three seasons, after that "plantbender" episode, I was wondering why earthbenders couldn't metalbend. Metal is, after all, an earthen mineral. So this episode really made me smile.
wunnup
The show did a fantastic job explaining the ins and outs of it while showcasing why Toph was the only one that could do it.
I wish they did the same thing for Bolin and lavabending
JDWalker
Hama’s line “There’s water in places you’d never think about.” can be applied to all the elements. There’s a character named Yun in the shadow of Kyoshi who could bend paint by using the earth minerals inside. Skilled enough benders can find bits of their element in all kinds of things.
Spiritx
There's a reason why she's one of the most respected fighters/benders in the show.
phantom
Metal bending was practically being teased throughout the series up until that point, and even IF it had been a case of deus ex machina, it would have been one that made sense, since at the same time Toph was discovering Metal Bending, the Guru was telling Aang about how metal was simply petrified Earth, it was giving both the characters and the audience a huge realization.
christianjohnson
Metal bending was so cool that when I was a kid I tried to learn how to so it by beating up a folding chair in my moms basement. Somehow I lost lmao
Hatneas
My favorite reason for Toph being the first to try metalbending works very well with her earth sensing. Ask any Earthbender if you can sense the earth around you, and they would say its impossible. But since Toph never had an Earthbending teacher outside the badger-moles, no one ever told her that. She pushes her bending to do seemingly impossible things because no one was ever around to tell her they were impossible.
zuulmeister
Another detail I love is how when Toph makes the metal armor, she doesn't need the usual opening for the eyes, making it a complete protection compared to regular armors.
thibautisserant
I think the waterbending the plants bit actually foreshadowed metalbending quite a lot, considering that it proved you could technically control something that isn't directly that element, but has components of that element.
TheMilitantHorse
This really heightens my belief that korra should’ve learned at least seismic sense before moving onto fire bending