This takes place during the episode, "Siege of the North." When Zuko and Aang are in the cave. There is a really big reference to "Bato of the Water Tribe."

"Welcome back," Zuko said as Aang woke up.

"Good to be back," Aang replied as he took a breath and prepared to airbend.

The exiled prince braced himself, but the wind never came. He looked to Aang in confusion.

Meanwhile, Aang was thinking about something from a few days – weeks? – ago, that involved Zuko.

"Come here," Aang said suddenly. Zuko hesitated, "Don't worry, I'm not going to bend or anything. Avatar's word."

Zuko looked on in confusion as Aang spoke up again, "It's really important."

Zuko reluctantly went over to Aang, who hopped up to his feet.

"I said, 'Don't worry,' I'm still tied up, after all." The boy said when Zuko paused mid-step.

"Closer." Aang said as he stood on his tippy toes to get on the same level of Zuko, "Lean down a bit, would you? I have a present from Katara, I promised I'd give it to you."

Zuko almost jumped back, expecting an attack, but Aang stopped, "It's not going to hurt."

Zuko leaned forward again, and Aang twisted to get to Zuko's cheek.

The arrowed boy firmly planted his lips on Zukos cheek, shocking the older boy greatly, "That was to thank you for returning her necklace." Aang said cheekily.

Seeing an opportunity as Zuko stared in confusion, the monk dropped to the ground and crawled worm-style out of the cave.

"Hey, this is some quality rope." Sokka said as he untied Aang a few minutes later.

Ah, well, there it is.