Fire is a chemical reaction that occurs when flammable objects combust and produce heat. Three elements are necessary to ignite and sustain a fire, fuel to burn, heat to ignite the fuel, and oxygen to sustain the fire. These elements are often represented as a fire triangle.
Fire requires a heat source to ignite and support the combustion process. Lightning strikes are a type of natural heat source, however, the majority of bushfires start because of human activity. This includes poorly managed campfires, damaged powerlines, sparks from power tools, and arson. A lot of effort has been made to reduce the number of human-caused ignitions, though it is not possible to eliminate them completely.
There are many different materials which can fuel a bushfire, the majority being forests, grasslands and other types of vegetation. When a bushfire meets an urban area, the fire will spread using any combustible fuel that is available. This might include cars, wood heaps, sheds and houses as well as trees and other vegetation in the area.
Fuel combusts when the heat source makes it hot enough to ignite. How easily the fuel ignites and (and then burns) depends on the type and amount of fuel, the shape and size of the fuel source, and its moisture content (the amount of water the fuel contains). Wet fuel will not burn, although it can be dried out enough from the heat of a bushfire to ignite.
Oxygen is necessary to support the chemical reaction (called oxidation) that causes fuel to burn. When fuel burns, it reacts with oxygen from the surrounding air, releasing heat and generating gases, smoke and embers.
Heat, fuel and oxygen make up the fire triangle