Water, earth, fire, air

Long ago the four nations lived together in harmony
Then everything changed when the fire's nation attacked
Only the avatar master of four elements could stop them
But when the world needed him most he vanished

A hundred years passed my brother and I discovered the new avatar
An air bender named Aang
And all those his air benders skills are great,
He has a lot to learn before he's ready to save anyone.

But I believe Aang can save the world.