Okay, this is what I have gathered from watching the shows.

Air doesn't have a secondary element as far as we know.

Earth has not one but four elements to bend. They can bend solid earth, sand, crystals, and metal.

Fire so far has been able to bend Fire and Lightening.

Water has been able to bend three things thus far. Water, Ice, and Plants. WHOOT!

Anything else you guys have noticed?

I don't know if this counts, but Aang and Katara did bend the clouds in the episode with Aunt Wu.

Also crystal is just solid earth in a geometric shape... I was wondering about the sand thing though. Sandbenders are specialized, from what we know. They can whip up the small sand particulates in a manner that reminds me of airbenders, even though we know they are actually a brand of earthbenders. But I wonder how effective a normal earthbender would be at bending sand. (Taking into consideration, Toph bent the sand slightly so that she could have a firmer foothold while she was holding up the library tower. But then again Toph is the world's greatest earthbender and she's the one that finally bent metal.)

Far as we know, 'secondary' elemental bending has either come from an isolated location or a complete mastery of the bending arts.

Toph, because of her innate sense of the earth and amazing mastery of earthbending, has developed her bending senses to the point where she can bend metal. I think(hypothetically) that this came about because of her lack of sight. It's a well-known fact that people without one sense are sharper with their others, so I think Toph's loss of sight made her even more in-tune with her 'bender-sense', the one that lets her feel the earth. This super-sensitivity allowed her to sense beyond just rock, and 'see' and bend metal as well. It's just my(rather simplistic) thoughts.

Then plant benders came about because the waterbenders of the swamp were around plants so much that this same 'bender-sense' became attuned to the plants, allowing them to bend those as well. The pole-based benders cannot do this, because of the lack of plantlife in their respective areas. Sandbenders, in much the same way, have senses naturally tuned to the sand because of being around it so much. Toph, because of her all-round better bending senses, was able to do it as well though, showing that mastery can overcome differing locales.

Cloudbending came about because of a mix of two elements- something normal benders cant do, so I won't go into that.

But following my hypothesis, maybe earthbenders who spent their lives around mines or metal-rich areas would grow up naturally attuned to metal and gems? That could be a possibility.

This is just how I've always thought of it.... sometimes I wish the Avatar creators would be up-front about this sort of stuff.

Interesting theory but I think anyone can bend these elements if their bending style permits it. I mean, Aang didn't spend to much time underground and yet be made a suit of solid crystal. I think anyone can metal bend if they spent the time to be intuned with it. The swamp benders did it because they were so close to nature (Being in a rain forest like swamp) and the sandbenders can do because they live in the sand. Basically, they adapted! A WHOOT for adaption!

I think the reason the waterbenders were able to bend the plants was because 1. they were close to nature, nd 2. because they bent the water inside the vines(i think they said this in that episode, but im not sure.) Anyway, the sandbenders are probaly because they adapted! Another WHOOT for adaption!
I just really wanna know what a 'secondary' element would be for airbenders. Thats like one of the best bendering things(i think) so i really wanna know. like i said before, it might be sound, but im not sure, any suggestions on the airbending element.

Thats what I was saying. Their bending abilities adapted themselves to the environment, allowing them to control subsections of their element that is normally out of a bender's grasp, or that most benders would have trouble learning.

I dont count Aang in any of my theories. He's the avatar, so had amazing adaptive powers unique to himself, and the ability to mix bending elements(so clouds, lava, etc are in his grasp). He's the exception: that is the idea behind the Avatar.

Toph is a bit of an exception herself; her bending senses(which I believe all benders have, btw: its the sixth sense that allows them to mold their element) are extremely fine-tuned and well-developed due to her blindness and reliance on it for 'sight'.

Well about sand bending, as I can recall from the episode in the library, it shows Toph bending sand, without much aim. She lets go of the building for a brief amount of time to send an ill-placed wave of sand at the sand benders, and as far as we know, Toph has spent little to no time on the sand because she can't see there. I think the fact is that benders can always bend their secondary elements, but some depend on the environment around them in order to be able to bend the certain things, like how Katara has to carry around the water in a pouch if she isn't around any water source.

Edit: Another reason we don't know the secondary element for air is because, as the series suggests, Aang is the only air bender left, and there is no way for him to encounter the benders of the possible secondary element, and he was not taught about the secondary element, seeing as he was "frozen away" for a hundred years.

Umm, as far as I know the secondary abilities are
Fire Benders make great blacksmiths and Bend Lightning (By the way the blacksmith thing was a joke)
Earth Benders bend Metal(But this requires a high level of mastery) and Sand (Yeah I think this requires some form of adaptions to be able to master) and Crystals (as someone else said they are nothing but condensed earth blah blah blah)
Water Benders Heal and can freeze water and plant bend
Air benders can yell really loud. Atleast I think thats what Aang did. maybe he just shot a huge gust of wind outta his mouth. Oh and lets not forget the Air Scooter lol

I think these are all the secondary elements (not sure):
water+earth=mud
water+fire=steam
water+air=mist
earth+fire=lava, ash
earth+air=dust, sand
fire+air=smoke, heat

Well as far as the water bending thing that is just basice science. All living plant life contains water. The plant life in a jungle usually contains more water because of precipitation.This means that the ability to bend plants would come as pretty much a second nature as they know they can bend water.

Then there is the water to ice thing. You guys are making this out to be something completely different than what it is. When water is frozen it becomes harder because its molecules come closer to gether causing its mass to increase.And vice versa. Then there is water to steam. This is done by heating it up wich spreads the particles of the water to make it a gas. If weather permits then it would also be possible for a water bender to
re-create the nature of this situation as they can move water. They can move the water molecules together wich if the weather permits, as I said above, and the temperature is at or below 32degrees Farenheit or 0degrees celcius, then the particle would not fight the water bender but would easily subdue to the Benders will as long as it is within the laws of nature.

All benders are bound by the laws of nature. There is no defying it. Yes Aang can fly but as natures law states, what goes up must come down. Yes water benders can bend water molecules to get closer in turn raising the mass and inturn making it a solid (Ice), but if the weather is hot then the ice will begin to melt as soon as it is bent into ice.

First off when water freezes, the molecles do not get closer together. Water is the only substance that expands when it freezes and becomes less dense, not denser. That is why Ice floats, Ice is less dense than liquid water.

If water wasn't so special then the bottom of ponds, not the top, would freeze over, killing all the aquatic plants and eventual freeze to the top until the entire pond was solid ice, killing all the fish. This is why the bottom of the ocean, which is under tremendous pressure and compress is still a liquid and not a solid. Without water being as special as it is, life as we know it would have never come into being.

Bender can't be possibly be bound by the laws of nature because according to the laws of nature, i can't move rocks just by stomping the ground or shoot fire from my hands. good thing too because if i could shoot fire out of my hand oh boy no one would be safe.

Then how does water become a solid? I could have sworn that molecules expand when their a gas, they start to crowd when its a liquid, and then they start to hug together when its ice? I could have sworn that was how it was done. Can someone please help me out here? The guy that left made my head hurt....

And the guy is somewhat wrong. Bending goes under laws of nature but not OUR laws of nature. If there were no laws to bending, then every thing would be bendable or not bendable at all. I like at how people go under the laws of our nature and don't think outside our box. Its fun and secluded.

That is how it normally works, Yoshiro, but not with water. Water is special. When it becomes ice, the molecules arrange themselves in a pattern, side by side, whereas when they are water, they are jumbled close together. By arranging themselves, it expands its area. Its odd, I know, but its the exception here, not the rule.

And he's making a point there with his last paragraph as well- he's saying we cant use the laws of nature, at least not all of them, since being able to bend is a violation of those laws itself. I agree with him, at least on that point.

the laws of nature are weird! let's just leave it at that, lol. And how about we keep our laws of nature out of A:TLA, it's starting to kill the show! ;D

Honsestly, I don't think there is anything special about sand benders. I think they just adapted to bending smaller paricles of Earth. Toph probobly can't bend sand because she is adjusted to moving huge boulders. Any normal Earth Bender would stink at bending sand. Toph can move the Rocks beacuse all of the particles that are packed tightly together. But if they aren't all jammed together to make on object it would be like bending four million rocks at the same time.

I think that sandbenders are adapted, but I think they could bend other earth stuff, given the time to learn it. An earthbender who has spent most of this time bending heavy rocks could bend sand, if he spent the time to do so. Toph bent sand somewhat durign the desert episode, though it was hardly effective.

Ya, there are different styles of bending that all bending types are adapted to... like foggy swamp waterbenders!

Well, I gave this secondary element thing some thought, and came up with pretty much what you folks have already thought of:

Fire- lightning
Water- ice
Earth- sand and/or metal, whatever

And, as predicted, I got stumped on Air. But see, I now have some kind of answer for that, and am willing to put it up to be debated on. I personally think that Airbenders' abilities alone are able to add to the physical abilites that even non-benders have, i.e. simple fighting moves like punching and kicking and the like. Air can add speed and power, and make the bender's weight less by making the air beneath him or her push him or her up, as Aang has done many times. Not flying, really, though I think that in the Avatar state Aang could actually do that.

But the reason I believe that Airbenders exclusively can do this is because, while water and earth benders can use their elements as a sort of transportation, they can't really use their elements to strengthen or make faster their movements.
Another way to look at it I suppose would be to take a few non-benders with learned ninja-like skills (kinda like Jet, maybe?) and give them the ability to bend one of the elements. You'd see the airbender simply magnify the physical abilities he or she already had, right? The other benders (water, fire, and earth) would just modify their usual technique to add in their new bending ability, right?

Just my opinion. Other thoughts?