Okay, so, my theory is that dragons eat a lot of fire producing things and flammable substances. (i.e. oil, charcoal, gunpowder, stuff like that) so they synthesize those substances into two basic components. Water and an alkaline metal. Alone those are harmless. But when in contact with each other, they're explosive. My idea is that they have two sacs in their throats that each hold one of the two harmless materials. When mixed, they open their mouths and allow the resulting explosion a way out. Now, there are five possible bad outcomes to this, though, all of which are kind of circumstantial. I'll tell you them if you like.