Well, when the show first started off there were four elements: Water, Earth, Fire, and Air. Later on we learned that Lightning could be bended (only those possesing the skills of a fire bender are capable of this mentioned by Iroh). Following the trail of elements, only one of the chineese dogma remains unnoticed.
Metal. Throughout the episode "the drill" various events led me to believe that a new element would be introduced. Such as Aang wishing he were a "metal bender" while he was trying to cut though the top of the drill with water. And the biggest event was when Azula was sliding down the drill after being thrown by the sludge water stuff, she stopped herself mysteriously and sparks flung from her shoes. (that could have jsut been for the steel in her shoes or something) However. She did stop herself on the bottom half of the rounded drill, so unles she had magnets in her body she would ave definately fell like aang. (he was aided by his airbending and Momo. but this is a fourum and i'm totally open to debate and other ideas.
