Fire and Earth.
The first thing he understands is Toph is nothing like Aang or Katara. She is impatient, wild, and brusque. Working out a rhythm between their bending takes weeks. The rhythm between life and stone should harmonize, instead they repel each other. The first time they attempted this, they both ended up with bone deep bruises.
He pushes inner fire into the stone and Toph shifts, before him is sand the color of emeralds. Stone bends and twists, fighting him like a wild ostrichhorse. He digs down deep, and demands the stone to submit. He is sweating and shaking, black corners at the edge of his vision becoming larger. He’s done it. The thrum of stone in his veins aches. The stone shifts under his fingers, connected to him.
Toph looks just as tired as he is. She has insisted to learn to bend as he does. He has no answers for her, so this is the best he can offer. The mountain burns under his feet, a pure life vein and it makes him edgy. It’s not nearly as bad as the Spirit Oasis that had just been frightening.
He places his right hand where the vein is closest and is gone. He is the mountain now. He sees the denning foxdeer yipping to their brood, the trees at the southern bank just waking up; he sees it all and more. His consciousness is aware of every branch and creature on this mountain. A sharp jab of pain hits straight between his eyes, and he is back. Toph is looking at him, fear written all over her face, and tears are running down her eyes. Shame wells over him, and he apologizes profusely for his mistake.
That had been stupid. This was only the second time he’d tried to perform that trick. Toph was an earthbender; of course she would have sensed his mind. His mind tips and sways, black dots appear before his eyes. He had wasted too much energy. His inner fire felt sluggish, the energy feeling like overly thick syrup in his veins. He just manages to get to his bedroll, and dreams of warm light and his mother’s soft voice