The breath weapon of a dragon is not a magical thing that spread out of no-where but had a more scientific explanation. When we eat our body by digesting created a gas know as Methane (CH4), dragon unlike human and other animal store this gas into another kind of "lung" that will serve as a bag/canister to held the gas that will be later mixed with a small amount of phosphor (P4) that has the propriety to ignite in fire at the contact of air. When the dragon wants to breathe fire, the methane is release into the lung and when the gas is in the air, the phosphor ignites and also puts the methane in fire.
Some dragons 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, I suppose to be Nitrogen (N2) (I have not yet found the exact composition of it but I continue to work hard to find the most probable natural refrigeration gas), 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. When a dragon needs to freeze an opponent, the highly compressed nitrogen, that almost reach the liquid state, is release in the lungs and when the gas comes into air, it uncompresses at an unimaginable speed. That result in the gas absorbs all the heat in is environment. This cause the temperature in the breath to drop to an average of -50 °C (more test had to be made to know a more exact °T), anyone caught in the path of the breath without heavy heat protection are at least seriously injured and only death remain for the weaker one.
It may have another explanation that can be explain by extremely efficient and fast oxidation-reduction reactions but we will not explain it much (everyone who has done is advanced inorganic chemistry should understand the concept otherwise you should review or have listen your college class)
The easiest of them, all organic creature secrete gastric acid to break down the food, dragon who can spit acid have a special organ that produce a powerful acid (probably HCl) that the dragon release when he breathe, the opponent is burn by the acid, even with the best armor usually make of metal that make bad reaction with acid.
I made one observation during my research:
it seems to have a direct link between the breath and the food, if a dragon
is starving his breath weapon doesn't regenerate. All the incredible system
of dragon makes it the most fascinating creature in the know universe.