Toph was training Aang in earthbending, reminding him that he had to be “hard as a rock.” However, the avatar took it quite literally, developing a rock-hard erection that excited Toph. So, she created a haven where she and Aang could have privacy. There, she began giving him oral sex, lubricating him, and then having him fuck her.
They were just getting started when Katara called the haven. To justify her moans, Toph invented a new kind of training. Katara believed them, but as soon as she left, Aang released his avatar state and proceeded to fuck Toph hard, using his bending to create handcuffs that would keep her immobile until he cum inside her.
The force was so strong that Aang broke the stone walls. Of course, Katara returned to them, and Toph quickly managed to invent that being naked helped one have a better connection with the earth.