You don't know? [ She's not sure how Mabel could have gone on so long not knowing what earthbending is, so Toph actually looks surprised at her confusion. ]

Earthbending lets me control the Earth, but I don't know why it's not working. It's like there's something wrong with the dirt in this place. [ Not that she actually knows where 'this place' is, but she'll worry about that later. ]