No don't worry this isn't one of my famous theories about how stonehenge was actually built by dragons, this is actually a pretty plausible thoery on how dragons breathe fire.

ok so here goes.

you know how there are some elements (like sodium for example) that are highly sensitive to water? im talking about the ones that can't even come into contact with air without causing a chemical reaction... well, if it was heated enough to become a gas it would be even MORE sensitive to moisture.

ok so there are the facts, now for the theory.
if dragons somehow held the gas of one of these elements in their stomach or lungs, when they breathed or burped the gas would come into contact with the moisture in the air causing a chemical reaction where the element would "sizzle and burn" for want of better words.

so... there is my theory! good no? i admit it has some flaws but i only thought it up about 10 minutes ago so i still need to perfect it, but its still pretty good huh?! xD