Everything in nature has a [i]point of luminescence[/i]. Fire, the result of chemical reactions, releases gas that has enough energy to reach this point. Theoretically, anything in nature can reach this point if enough energy is applied and then it will start to glow. The same reason a bar of steel glows red when heated is the same reason fire glows red too. The only difference being that fire is a gas and steel is a solid. [spoiler]fun fact: Humans are already hot enough to reach the point of luminescence. BUT the visible light we produce is so dim that our eyes cannot detect it.[/spoiler]