What Is Fire?

Even before the atom debacle, a huge idea was that there were four elements. If you mix fire and earth, you get metal or whatever. And in this description where there are only four elements, everything else is just combinations of these four elements. It's kind of intuitive because you take four things that hardly share any properties at all or no properties,. You have one thing that flows and, you know, is a liquid. One thing that is invisible and is a gas. And then fire, what, what? Fire.