-lolsnip-
Lol'd
*WARNING BIG WALL OF TEXT*
But seriously, dragons evolved from snakes, which is why they have a poison fang, but they don't poison their enemies, the liquid in the gland for the fang is actually a flamable liquid (i.e. liquid gas or something). But how do they ignite it?
They have a special pouch in their jowl (I think) that holds a peice of flint, when they ignite the flint with something (I suspect the underside of their tounge is rough enough to do this) they squirt the liquid out, thus igniting it and, volia, fire.