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If one of these is not present, the fire cannot start. If one of these is taken from a fire it will go out.
But how does this all work? Here comes the science bit - concentrate.
Oxygen
We all breathe Oxygen (O²) everyday - in fact, without it we would suffocate. But did you know that fire breathes Oxygen too? And, like us, without Oxygen a fire will also suffocate.
When Oxygen in the air combines with flammable vapours given off by Fuels they create a form of heat at a molecular level. Then, a source of ignition (a match or spark, say) can cause it to combust.
Without enough Oxygen, ignition cannot happen. In the opposite way, if there is too much Oxygen then the vapours won't be concentrated enough to ignite. The ratio of vapour to Oxygen is known as the 'explosive' or 'flammable' limit and is different for each gas or vapour.
Heat
Combustion occurs when flammable vapours mix with air (Oxygen) and are ignited by a spark or flame.
Solids give off flammable vapours by being heated. Certain solids such as paper or flour appear to ignite almost instantly. This is because they give off vapours and reach a flammable temperature almost immediately. In fact, fine dusts dispersed in the air can explode because they give off vapours and ignite so quickly it appear to happen instantly.
Other solids like timber take longer to ignite because they are more dense and so don't give off flammable vapours so easily.
Liquids are a bit different to solids. They are a lot more runny for a start! But, where solids need to be heated to give off flammable vapours, some liquids give off vapours even in cold weather.
The name for the temperature at which vapour from a liquid can be ignited is known as its 'flashpoint'. The flashpoint of petrol is 43°C below zero. Meaning that even on the coldest day you can possibly think of, petrol can still ignite very easily.
Thermometer showing 'flashpoints' of liquids.
Gases are already vapours and will ignite in a single stage.
Fuel
So, in our fire triangle we've got Oxygen and Heat, but we also need something that will burn - this is our Fuel.
Fuels can take almost any form:
Solids like wood, fabric, rubber and plastic.
Liquids such as petrol, oil, cooking oil or even nail varnish remover.
Gases like propane, butane and 'natural' gas.
Different fuels burn at different rates and with different intensities. Some types of fuel, under certain conditions, can burn at over 1000°C! That's hot enough to melt Aluminium! The table below shows the temperatures that some types of fuels reach when they burn.
By contrast, the temperature of the surface of the Sun reaches around 6000°C!
Table showing burning temperature sof various fuels
