Do I need to spell it out?
Sometime in the evolutionary horn development of dragons, they succeeded in harnessing their natural gut bacteria in two unique ways, flight and fire. Many animals possess in their guts active bacteria which aid in the digestion of food. The bacteria inside the dragon's gut produces hydrogen. The hydrogen produced in the gut is then transferred into the internal bladders. Dragons haves several characteristics typical of flight. The honeycomb structure of their bones allows them to be light, while retaining their strength. Huge hearts are also typical of flight, as chest muscles would need vast amounts of oxygen-rich blood in order to move the large wings. As well as the flight bladders being used as buoyancy aids, they also double as fuel stores for the fire that dragons are famous for breathing. The hydrogen and methane combine with oxygen in the presence of platinum, as a catalyst, and combust. The inside of dragons mouths are armour-plated and it has a false palate in its throat, similar to a crocodile, to stop backdraft. Because of the dual use of the bladders, the more fire a dragon breathes, the shorter the distance it can fly.