Re: If fire is energy and has no mass, why does it take the shape it does.

Heat travels upward due to its convective currents. Fire, having little or 
no mass in itself, travels the path of the convected heat currents, hence 
the shape of the flame.
It doeson't get any simpler than that.
Remember, however, that fire is defined as a self sustaining process of 
combustion which generates heat, light, gases and smoke. The only one of 
these things which is truly massless is heat, Light has a very small 
amount of mass according to the theory of relativity, albeit 
immeasureable, it is still implied.

Thanks for the question.
Donald e duggan