Zuko’s breath of fire in Book 1 v Book 3. I think it’s fair to say that in Book 1, he still hadn’t quite mastered proper breath control but, by Book 3, he has completely mastered it.

Notice how in Book 3 Zuko takes very deep and controlled breaths designed to take in more oxygen while in Book 1 his breaths are very short and quick. Also notice the color of Zuko’s fire. In Book 1, Zuko’s breath of fire is red, meaning that he’s feeding less oxygen into his flame, making the fire somewhat cool. In Book 3, his flame is yellow, meaning that he’s able to feed more oxygen into his fire, which consequently makes his fire hotter.

Now, you can argue that this discrepancy has to do with the fact that in Book 1, Zuko was in the tundra. HOWEVER, in “The Boiling Rock,” Zuko was being kept in the Cooler, which was supposed to prevent firebending. Not to mention that when Chit Sang was in the Cooler before Zuko, we saw him shivering and evidently being very cold. Zuko, on the other hand…