The Theory of the fire-spit of a dragon

the thing that makes the dragon most feared is their ability to breath fire, Even though, not all dragons breath fire. Some have freezing cold breath, others have acid, and some don't use their breath as a weapon at all! The two that have fire breathing power are the Red Dragon and Fire Drake

There are many theories that attempt to explain the fire-breathing ability that most western dragons posses. Usually the mechanics of the ability are ignored completely, or waved off as being magical in nature.  However, there are many very scientific theories surrounding breath of fire. for example, Perneese dragons must chew a phosphoric-bearing rock, and it enters their second stomach to be changed into a substance that ignites when it comes into contact with oxygen. 

Theory 1

the first theory is what i've been reading mroe about is the fire-breathing ability with the "blimp" theory of flight.  In this theory, dragons are filled with hydrogen, so they don't actually fly, they float.  It ties into fire-breathing because dragons can just expel the hydrogen and ignite it somehow, sometimes creating a chemical reaction, or creating a spark, usually by grinding their teeth together. 

Theory 2

the second theory is the hydrocarbon spit theory. this is more scientific and convincing than the first one. 
This theory states that after a dragon digests its food, and after it passes on into a second stomach where it is
digested further and turns left over food acid into chemicals which are
useful in producing fire.

Inside
a dragon, near the second stomach, are several fire glands. Inside these glands
there are different hydrocarbons, such as methane and octane. The glands are
muscular structures and connected to the nervous system, so the dragon can
control when it spits out fire.

The
dragon also produces an oxidant agent and a strong high smelling, highly
flammable fuel. So when the dragon opens up the glands it lets out hydrocarbons
which mix with the oxidant agent and fuel, creating the beginnings of a fire.

By the
time this fire has been let out of the mouth or nostrils, it has become a
strong and powerful blast. There are many species of dragon that are able to
control with great precision the shape, temperature and duration of the flame.
This depends on how much control over fire glands a dragon has.

The Theory of the ice-spit of a dragon

Another type of dragon is the one that can breathe a cone of frost, the explanation for this resides also in the food that the dragon ingests. The food is break down into the stomach primary for nutrition but the remain bear some chemicals reactions that will give a gas, which is Nitrogen, the gas is compress by extremely strong muscles, exactly alike the base system of a refrigeration system, the dragon doesn't need to thing for it because it is spontaneous and painless. 

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The Theory of the acid-spit of a dragon

the last type of dragon is the one that can breath acid. this is the easiest to explain because all organic creature secrete gastric acid to break down the food, so the dragon who can spit acid have a special organ that produce a powerful acid, that the dragon release when he breathes, the opponent is burn by the acid, even with the best armor usually make of metal that make bad reaction with acid.

For other breath weapon least know follow the same principle and don't need much detail about them.
it seems to have a direct link between the breath and the food, if a dragon is starving his breath weapon doesn't regenerate.