Avatar: The Last Airbender/WMG/Science of The Avatar World

Draft animals are about to be replaced by motor vehicles and aircraft in a big way.
The Fire Nation's tanks and military airships had do have been made somewhere. All those factories just can't close down- on top of all the returning occupation troops, the result will be mass unemployment. The quickest way forward is to develop civilian vehicles and create demand for them.
 * They can start with selling a fully-enclosed cargo van to a certain cabbage merchant...
 * Selling one? The Legend of Korra suggests the Cabbage Merchant built one.

The Avatar planet has a lower gravity than our planet.
That's why every last person can do tremendous awesome backflips and sommersaults. Not that it's not still difficult, but it's not quite as hard. That's also one of the reasons blunt damage doesn't do so much... damage.
 * Maybe becuse the Avatar planet is also smaller than our planet? Would make sense with how we can sometimes travel a long distance in a relatively short amount of time.
 * But I think it's just because we have kung-fu physics.
 * I'm pretty sure lower gravity would create more problems than it solves. On the other hand, chi is the catch-all handwave for Charles Atlas Superpowers in Asian and Asian-based fiction.
 * That's not how momentum works at all.
 * It would also explain how battleship-sized armored zeppelins would stay up using nothing but hot air, and how effective steam power seems to be in moving very heavy looking vehicles.

The animals in the Avatar World used to be regular animals...
...but at some point they all turned into/were replaced by the current ones. What makes me think this? Notice that almost all animals in the regular Avatar World are Mix-and-Match Critters, but all of the animal spirits from the Spirit World are regular animals (unless you count Fang). How did it get like that?
 * It would also explain why the name of almost every animal is simply the combination of two animal names (such as Platypus-bear). With the normal way languages evolve, each animal would get it's own name, not the combination of two names of animals which were much more rare. And it would also explain why the Earth King has a regular bear as a pet: regular animals would be extremely rare, and having one a sign of wealth.

If All animals are Mix-and-Match Critters, than Avatar Humans are...
I say human/beluga whale, seeing how hard it is to burn or damage anyone in this world.
 * Frog-lemurs. Giant frog-lemurs. Or normal-sized frog-lemurs, and everything else on the Avatar world is a pygmy species (including the asteroid-planet on which they live), aside from the badgermoles, anteater-moles, and lion-turtles.
 * But not all animals are mixed. Bears and wolves come to mind.
 * Don't forget Miyuki.

Sozin's Comet is actually a rogue star
Seriously. Real comets are masses of rock, ice, and gas. They'd power up everybody but firebenders. But Sozin's Comet, like the sun, jacks up their power to Ludicrous Speed. Plus, the glow fills the skies when it shows up, turning the world red, and it's departure looks like a setting sun. Therefore, Sozin's Comet is a Brown or Red Dwarf (the star, not the show) that got captured by the sun's gravitational field, with an orbit that crosses the planet's every 100 years.
 * On-screen, the comet ignites when it hits the atmosphere of the planet, so that's why it powers up Firebenders. It's not the greatest explanation, but it's a helluva lot less problematic than having a star touch a planet's atmosphere.
 * Maybe it's a matter of perception. They think that what appears to be a giant fireball in the sky will help their bending, so it does. Following that train of thought, waterbending isn't actually affected by the moon being out, but people think it does, while their minds actually make their bending stronger. Same with firebending and the sun. Your Mind Makes It Real, indeed.

Aang's staff is somehow special
Aang's staff seems to be made of ordinary wood, but we have seen several times that it can break through solid rock or burst rocks that were thrown at Aang. This is rarely a result of Aang's airbending. So how does he do it?
 * Obviously it's made of adamantium. The whole "wood" thing, it's just a cover-up.
 * So it needs to be made out of something flamable but otherwise completely industructable. Hmm...of course! It's made out of Twi-pires!
 * It can't be made of adamantium, it's a delicate instrument (like Aang). What's flammable, like Aang, and connected to the Avatar? The mummified remains of a previous Air Nomad avatar. Or avatars, considering the area of the wings and the surface area of an average human. It's not like blood-coloured bone hasn't been confused for wood before. Being passed down from and made of previous avatars lets the Avatar channel energy through it with more efficiency, though not necessary. It's likely that the Earth, Water, and Fire nations have their own similar artifacts, that work well with the first element the avatar born in that nation is likely to learn.
 * That works up until Kyoshi's fans. What sane person thinks to themselves "a fan is a perfect tool to bend earth with"?
 * Who says that was her Avatar artifact? Would a giant stone drill fit her philosophy better? I think not. And remember, it's not integral to the guess that all nations have their own Avatar artifact (that was just an extension of the identification toy things posessed by the four nations).
 * A metal fan. Besides, The Man Makes the Weapon.
 * Anyone else notice that Teo's wheels are made of solid wood? In Real Life, any wood light enough to be practical for wheelchair (let alone aircraft landing gear!) purposes would be too soft; anything sturdy enough would be far too heavy. Aang's staff is probably made of the exact same wood, just stained a different color.
 * Sapient pearwood, anyone?

Sky Bison are about to go extinct.
Appa is the Last of His Kind, isn't he?
 * Considering the crazyness of the rest of the world's animals, this could be averted with some Hot Skitty-On-Wailord Action!
 * Waterbenders learned from the Moon and Ocean, were all other benders learned from bending animals. Tui and La embodied themselves into physical forms, and Roku's dragon, which is the Fire Bending Animal, is clearly a spirit now as well. Perhaps, when it is safe, air spirits will re-embody and recreate sky-bison.
 * Jossed as of Aang's finding the island full of them between his and Korra's series.

== The world of Avatar: The Last Airbender is the product of a Godlike post-singularity intelligence who, having reshaped the Earth into the form we see on the show, descended into the bodies of the lion-turtles. == The Avatar world is designed to create a stable, enduring environment for homo sapiens to inhabit. Bending is meant to reinforce the socio-cultural aspects of this system and to act as a retardant on industrialization. After all, the Fire Nation's been fighting a world war for a century and they haven't invented anything much better than the same steamer ships they were using at the war's beginning? And, going by the number of statues we saw in "The Southern Air Temple", if the Avatar Spirit has existed for many thousands of years, how have the Four Nations stayed largely coherent? Sure, the Sun Warriors became the Fire Nation, but that's still a one-to-one transition.

The lion-turtle telling Aang about the pre-Avatar era of energybending? That's the lion-turtle explaining the dreamlike powers of a post-singularity civilization to a twelve year old pre-modern monk in as simple as terms as he will understand. Thus energybending is just the cheat codes to the Avatar world's source code. The Spirit World is a repository of other intelligences from this post-singularity civilization.

And consider the lack of high child mortality in the Avatar world compared to real world historical levels, as well as the incredible Olympic-level athleticism that can be found even in common peasants. The humans of Avatar obviously don't match up to what we would consider real world averages of physical strength, durability and speed. Why? Because the human race was edited a bit by the post-singularity intelligence during the re-creation of the Earth in order to minimize unnecessary suffering while not undercutting what would we might consider "basic humanity." The Avatar-humans think, talk, and act like us, but they have stronger and healthier bodies. They are idealized humans.

The Swamp doesn't actually show people who have been lost.
That's just Huu's interpretation of the visions. The swamp actually shows people with an emotional connection to the viewer that is not current - either because they no longer share the connection or because they don't share the connection yet. It's just easier to explain as "people we've lost", since most visitors see important people from their past. In other words, it's not that Aang knew Toph from a past life or that she will be lost to him, just that he hasn't met her yet but she'll be important to him in the future.
 * This would make sense because unless Toph is really going to die she should not be in The Swamp.
 * It may not be such a literal meaning of the word "lost". Since Aang saw an illusion of Toph in her girly dress, maybe it is more a reflection on how she will lose the fake personality she uses to fool her parents. With the fake personality "lost" she will no longer hide her bending from anyone.
 * Also included is "people we've loved". Combine with "time is an illusion," and you get "people we will love in the future." Taang-shippers may rejoice, but there are different kinds of love.

Pentapox is a realish disease in the Avatar world
Whenever pentapox is mentioned everybody's response is to ask if the person next to them's cousin died from it. This can only mean that while "pentapox" isn't a real illness there is one that either sounds like it or has symptom that look similar (at the end stages).

Pants really are an illusion.
Confirmed by dream!Aang and dream!Ozai in Nightmares and Daydreams.

The Average number of children per household in the Avatar world is 1.5.
Notice how the only person ever mentioned to have more than one sibling is Tai-Lee, a Fire-nation noble. Perhaps the fear of warfare led most families to only have a couple of childen to spare them the bloodshed. However, war does not seem to be the main factor since Aang, Bumi, Roku, Kyoshi, Sozen and Guru Patink all seem to be only childern. If anything this means the war increased the birthrate.
 * Mai also had a younger brother Tom-Tom.

Firebending burns calories.
After all, calories are units of heat. And it would explain why most firebenders in the series seem to be pretty ripped. (Iroh is the exception because his techniques are fueled by his own awesomeness - and possibly caffeine from all the tea he drinks - and therefore he isn't burning off as much of what he eats.)
 * Of course Firebending burns calories. All bending burns calories, because it involves so much movement.

Iroh used a passive form of firebending to burn calories faster while working out.
It explains how he was able to lose so much weight in what seemed like one day.

The Avater-verse contains dye plants that don't exist on Earth.
The Water Tribes must dye their clothing with something, yet there are few natural sources of blue dye, and the best-known (indigo and woad) don't grow in Arctic climates.
 * That's almost definitely true - red isn't a particularly easy color to produce with natural dyes from our world, either, but in the Fire Nation red cloth is so cheap that they dress prisoners in it. However, I don't remember seeing any plants at all in Water Tribe territories. My favorite theory is that the blue dye comes from a sea creature. It's not unprecedented - murex (the "royal purple" of the Greeks and Romans, among others) is extracted from shellfish, and squid produce ink. Probably the Water Tribes eat that sea creature and save up the dye glands - they'd have a pretty easy time keeping them frozen until they got used.

There are more lands yet to be discovered in the Avatar World.
Yes, I believe the map we see all the damn time is incomplete, and there are more lands out there that are undiscovered out there. This could possibly mean that there are more different societies out there in the world, possibly based on African, Roman, or Celtic societies. It does not mean they have different bending elements, for example, the Sun Warriors and their Aztec ways, but some might have others, like the over-used Shadowbending idea (which is a good idea if used right). Who knows, they might utilize this.
 * Possibly one of those countries is fantasy-counterpart India. Guru Pathik had to come from somewhere.
 * One day, people will flock to those other lands and come together to become a city known as Ankh-Morpork...
 * All the area shown on the map in the title sequence is located on one hemisphere of the Avatar world. The other hemisphere contains the European, African and American equivalents that have developed their own styles of bending and have their own spirits. Why the Avatar hasn't been born into these nations will be a mystery for a future series when the two hemispheres learn about each other and have to deal the others' existence.

Appa is not the last flying bison.
He's the last domesticated flying bison. If necessary, future airbenders might be able to seek out a similar wild animal with the same natural airbending abilities for lessons - a more agressive, skittish animal with none of the tame bisons' tendency to bond with people. This will lead to the rise - or possibly re-emergence - of a different style of airbending, one that emphasizes the unpredictability rather than the gentleness of air.
 * Confirmed, at least about Appa being the last Sky-Bison.
 * Given that sky bisons are not extinct, someone will almost certainly learn Airbending from them at some point in the future. So even if Aang doesn't manage to re-found an Airbending culture, chances are someone else will.

The world we see in the show is actually a terraformed Mars, in the far future.
It has been mentioned that the planet is small, and that gravity does not seem to be as strong (judging by non-bending acrobatics, falls, etc).

The "moon" we all see in the show is actually an artificial satallite, an orbital shield generator built in ancient times to serve a dual-purpose: generate an artificial gravity field to create a tidal system as part of the terraforming process; and as a byproduct of the massive amounts of energy being used to power the gravity generatores, it produces a large electro-magnetic field which protects Mars from solar radiation (Mars has no natural magnetic field, which on Earth protects us from the Sun). I mention this because it segues into the theory that Benders are powered up by various forms of radiation.

There are different subspecies of dragons.
From the dragons in the series, Ran and Shaw are noticably different from the dragons kept as companions. They have different physical features and are much larger.

The weird animals in the Avatar world are due to evolution taking a different course then on Earth
Assuming that Avatar is not set on Earth, animals would evolve differently. Badgermoles share a common ancestor with Earth badgers, sparrowkeets with Earth parakeets, and so on so forth. The seemingly hybrid aspects are the result of. This is why most animals are hybrids while a few others aren't. This isn't to say that are no hybrid animals, but rather why most appear to be hybrids.