Metalbending is thought to be impossible in the entirety of Avatar: The Last Airbender — right up until Toph creates the bending style, using skills only she possesses. Earthbending is technically the ability that is most capable of being too powerful in Avatar, as it can literally destroy the ground someone is standing on, so the show established early the limitations of these powers. Not only is much time shown explaining that earthbenders under Fire Nation control are punished with incarceration for using their powers, but it's also shown that they have no control over metal.

Avatar: The Last Airbender explains early on that metal is too refined and changed from earth to be used by even powerful earthbenders like Bumi and Toph — and that the substance is actively impossible to manipulate in the same way rocks and the ground can be. This is repeatedly shown used against earthbenders throughout the show, most notably when the powerful King Bumi was neutralized during the Fire Nation invasion of Omashu by trapping him inside a metal cage. Toph — the Gaang's talented earthbender — would also find herself trapped in such a cage mere episodes later, in season 2, episode 13, "The Earth King." The notable difference, however, is that Toph would be able to break free of the cage by inventing metalbending, something she would later popularize by training others in the art.

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While it may initially seem strange that other masters failed for so many years to do what Toph accomplished, one crucial thing differentiates her earthbending style from others: Toph has been blind since birth. As such, instead of physically looking around places, Toph grew accustomed to using her powers to sense her surroundings, meaning she spent every waking day channeling her connection to the earth element. This is why Toph is able to sense that the metal that she is encased in has small areas of unrefined earth in it; because every day for years upon years she has honed this specific skill known as seismic sense down to a fine craft — turning Toph Beifong into one of Avatar's greatest earthbenders.

 

Toph Metalbending

This said, it also stands to reason that Toph was more inclined to try to metalbend than others because her stubborn nature meant that she was less inclined to believe others who said it was impossible. Were it entirely a case of having insanely honed earth-sensing abilities, it seems likely she would have struggled to teach metalbending more later in life than she seems to have been able to — and as such, it also stands to reason that a portion of her success can also be attributed to her unwavering spirit.

It also doesn't hurt that her teachers were the badgermoles: creatures that are also blind, and also use their earthbending abilities to manoeuvre around. Traditionally in the world of Avatar, those who study their bending from the animals who first used it become far more adept in their powers, and so there's a final potential bonus provided to Toph, as she studied them immensely in order to perfect the art of using earthbending as a way to enhance the senses. With all this in mind, it seems downright impossible for anyone else to have started the practice of metalbending in Avatar: The Last Airbender other than Toph, as her abilities, her past, and her inherent nature play vital roles in allowing her to develop the skill.

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