Do you know why flames burn upward? Simple. Flames contain hot gases that rise because they are less dense than their surroundings. In fact, they contain streams of particles, escaping from the burning item. At the same time, there are other particles that move into the fire. This is combustion. The produced gases are hotter than the environment due to the generated energy. They expand, as heated gases do, and as their density is lower than the surrounding atmosphere, they tend to rise. And so does the flame!