His face when it actually flies.
Also, how is that so unbelievable? If aang proves he's an air bender and humans can bend elements why is it so hard to believe Aang owns an air-bending bison?
Well no one has seen a person bend air for 100 years, and Appa is the last sky bison. Seeing him bend air or Appa fly would be like seeing someone bending elements in the real world, I'd probably shit my pants seeing that!
Also, how is that so unbelievable? If aang proves he's an air bender and humans can bend elements why is it so hard to believe Aang owns an air-bending bison?