The three essential requirements to produce fire are :
                                  (1) a combustible substance eg. paper,wood
                                  (2) supporter of combustion i.e. oxygen
                                  (3) supplying heat so that the substance reaches it's                                               ignition temperature

A fire extinguisher is useful for controlling fire, since it contains carbon dioxide which is not a supporter of combustion.This carbon dioxide forms a blanket around the fire ,thus extinguishing it.

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