The three essential ingredients for a fire are fuel, oxygen, and heat. A small input of heat (from a match, for instance) converts the fuel into gas or smoke that can readily mix with oxygen in the air and start burning. Gas and smoke contain many compounds, but the fire fundamentally works on one simple chemical reaction. Hydrocarbon fuel plus oxygen yields carbon dioxide and water plus energy, which emerges in the form of heat and light. Heat from the flame liberates additional combustible gas, keeping the reaction going. The region containing enough fuel and heat to maintain this process defines the boundary of the flame.

This is thanks to E. Robert Fanick, a senior research scientist at Southwest Research Institute in San Antonio, Texas! In response to questions in Discover Magazine, 12/1999