I haven't seen the second and third book in a while so I could just be sounding dumb here. I just finished watching a clip of " The Dragon Dance" because it's one of my favorite scenes in the whole show and it got me thinking. We know Earth bending was passed down by the Badger Moles and that fire bending was passed down through the great Master Dragons where did water and air come from?
Here's the Dragon dance incase you guys want to see it again.
http://www.youtube.com/watch?v=opuEhcbmPww











