Of course, any comic or film that features super powers is going to need various plot devices that allow the impossible to happen. For the Fantastic Four, one problem that is always mentioned concerns the costumes: namely, why isn't the Human Torch's uniform burnt up whenever he "flames on"? Similarly, why does Reed's costume stretch along with him and how does Sue's costume turn invisible along with her body? To address these issues, "unstable molecules" (whatever they might be) are invoked. People are generally happy to accept that the uniforms are made from this convenient material, simply so the story can get going. But there are other problems with Johnny Storm's powers: problems that are so big they make his clothes not burning away seem reasonable by comparison. What's worse is that the biggest of these problems completely undermines his coolest ability, flight. In the comics, Johnny's flying is explained away by saying that flames are lighter than air. This explanation doesn't hold up to any scrutiny whatsoever. Regardless of what the flames are doing, Johnny doesn't lose all his weight when he catches fire. A burning man is no more able to fly than a not burning one since he's just as heavy. Compared to that, asking what exactly is combusting around Johnny's body and producing those flames seems a moot point.