The Real Answer Is Fire, I'm Not Sure.

Boltzmann: What is the most biologically physically plausible way to get a biological creature to breathe fire, that I also don't know the answer to. He says if you were able to design rather than just build it biologically, what you might want is to have two gases, which are separately, perfectly stable, but then when they're combined, they become highly flammable. That happens in rocket ships, right? Hydrogen and oxygen can be kept separate,. but then when you put them in the same region of space, they burn very, very easily. "I'm not sure I buy quite the difference between the two things," Boltzmann said.