Q&A regarding Avatar
 

I've got two:

Q: Why are the Na'vi blue?
A: Because of the high concentration of CO2 and other components in the atmosphere, hemoglobin, the responsible for oxygen transportation, turns darker; making the skin look blue. A similar effect can be experienced when cutting off blood circulation from a part of the body: cells begin to produce CO2 turning the skin of the affected area red, violet, purple and, finally, blue.

Q: Why can humans still breathe when there are bullet holes on their vehicles' windshields?
A: Pandoran air is denser than the air produced by these machines to allow their crew breathe. When a bullet hole is made, Pandoran air deposits in the lower stages of the vehicle, and slowly fills the cabin; however terrestrial air remains on the highest parts of it, making breathing still possible.

I hope this makes some kind of sense.

A. Because they're beautiful
A2. why not? :)

Human vehicles and buildings are pressurised specifically so if there is small damage, it prevents Pandoran air from immediately entering, instead causing a drop in pressure and loss of some atmosphere from the interior.

#2 the cabins are pressurized so when there is a small hole, earth air escapes.

I have one :D
Q: Why do all other creatures on Pandora have six limbs when the Na'vi only have four?
A: Who the hell knows? Evolution, it's a theory, therefore doesn't have to be justified :P

Prolemuris have four arms with two fingers each. This means that if evolution theory is correct, the Na'vi and them might have shared a common ancestor.

What's the deal with those na'vi playing basketball in the beginning? I didn't understand what they were doing. Are they Avatars too? If so, where were they during the whole story!?

Yep, they were Avatars too. The other Avatars were probably out exploring other parts of the forest. Though, in the script Max and the other Avatars take over Hell's Gate during the battle

Max runs through the command center with a bulldozer.

Q: Neytiri only has one ear peirced and guaged, why the right ear? :D

Yeah, and then a bunch of gun-toting Avatars jump in.:D