Book 2 Chapter 11 "THE DESERT"

Team Avatar is stranded in the desert, without sufficient provisions to make the trek out again now that they no longer have Appa. Elsewhere, Iroh reveals a mysterious association with a secret society when he uses a White Lotus tile as an opening in a match of Pai Sho, his favorite game. The move functions as a message to his opponent, who then helps protect Iroh and Zuko from Xin Fu and Master Yu, who decide to take a break from hunting Toph to try and collect the bounty on Zuko and Iroh. After a long trek and much hardship in the desert, Aang tracks down the sandbenders who stole Appa, and learns that Appa was traded to a merchant and is likely in Ba Sing Se to be sold. Upon hearing that Appa was muzzled, Aang becomes so angry and upset that he enters the Avatar State, destroying nearly everything around him, but is eventually calmed by Katara.