Fire is the visible product of combustion — an exothermic chemical reaction between a fuel and oxygen that releases heat and light fast enough to produce a glowing plume of hot gas and soot that behaves almost like a plasma. Because hot gas is less dense than the surrounding air, buoyancy drives it upward, and that rising column is constantly disrupted by cooler air being pulled in from the sides (entrainment), which is what makes real flames twist and flicker rather than burn as a smooth, steady cone. The color gradient you see mimics blackbody-like thermal emission combined with chemical excitation of reaction intermediates: the hottest gas near the base glows blue-white, cooling mid-flame regions shift to yellow and orange, and the coolest tips and trailing soot fade to red, deep red and smoky grey as they lose energy to the air. This scene approximates that physics with layered particle systems — a dense inner "fire" stream colored by a temperature-like lifetime curve, plus a sparser "smoke" layer of unburned soot particles drifting above the flame tip — combined with a flickering point light to sell the sense of a living flame.