A<unk>  is often defined as the visible (light-emitting) part of a fire. Physically, it is caused by a highly exothermic reaction (for example, combustion, a self-sustaining oxidation reaction) taking place in a thin zone. A<unk>  generally emits light, by two different mechanisms which will be described below.
The color and temperature of a<unk>  are dependent on the type of fuel involved in the combustion, as, for example, when a lighter is held to a candle. The applied heat causes the fuel molecules in the wick to vaporize.  In this state they can then readily react with oxygen in the air, which gives off enough heat in the subsequent exothermic reaction to vaporize yet more fuel, thus sustaining a consistent<unk> . The high temperature of the<unk>  tears apart the vaporized fuel molecules, forming various incomplete combustion products and free radicals, and these products then react with each other and with the oxidizer involved in the reaction. Sufficient energy in the<unk>  will excite the electrons in some of the transient reaction intermediates such as CH and C2, which results in the emission of visible light as these substances release their excess energy (see spectrum below for an explanation of which specific radical species produce which specific colors). As the combustion temperature of a<unk>  increases (if the<unk>  contains small particles of unburnt carbon or other material), so does the average energy of the electromagnetic radiation given off by the<unk>  (see blackbody).
Other oxidizers besides oxygen can be used to produce a<unk> . Hydrogen burning in chlorine produces a<unk>  and in the process emits gaseous hydrogen chloride (HCl) as the combustion product. Another of many possible chemical combinations is hydrazine and nitrogen tetroxide which is hypergolic and commonly used in rocket engines.
The chemical kinetics occurring in the<unk>  is very complex and involves typically a large number of chemical reactions and intermediate species, most of them radicals. For instance, a well-known chemical kinetics scheme, GRI-Mech 
, uses 53 species and 325 elementary reactions to describe combustion of natural gas.
There are different methods of distributing the required components of combustion to a<unk> . In a diffusion<unk> , oxygen and fuel diffuse into each other; where they meet the<unk>  occurs. In a premixed<unk> , the oxygen and fuel are premixed beforehand, which results in a different type of<unk> . Candle<unk> s (a diffusion<unk>  operate through evaporation of the fuel which rises in a laminar flow of hot gas which then mixes with surrounding oxygen and combusts.
Flame
Breathe odor
Odor
Itch
Deaf-mute
Wheeze
Stye
Hot flash