Fire is a rapid, exothermic oxidation reaction demonstrated in its simplest form by the Fire Triangle. 
If asked, most people could probably recall the three sides of the Fire Triangle from memory. It is, of course, the simplest pictorial representation of the components needed to create and sustain ignition, or the chemical reaction of combustion.
All three sides are equally important: they all rely on each other to complete the process. Without any of these, no chemical reaction could take place, and fire could not be generated.
The atmosphere of our planet is 21% oxygen and due it’s presence around us, could result in fire at any time if the other two elements are present. During combustion the fuel reacts with oxygen molecules, which makes water and oxides, a by-product of which is heat and light.
Fuels come in many forms, usually things you can find and see all around you and all have different flash rates, meaning they will ignite at different temperatures and will burn at different speeds once ignited.
Fuel is arguably the most dangerous element of the fire triangle, as each different type comes with its own requirements for storage to keep it safe from accidental combustion, due to oxygen being ever present.
Different fuel types are the main reasons for the variety in fire extinguisher types, an important aspect of fire safety. So important that it is referenced in the Regulatory Reform (Fire Safety) Order, which explains that premises may need ‘one extinguisher for every 200-metre squared (m2) of floor space with at least one on each floor, to ensure fire can be halted if it occurs.
Heat will help ignite a fire and keep it combusting. As the materials involved in the combustion process are exposed to fire, further heat is also generated as a natural by-product of the reaction, along with light.
The Fire Tetrahedron
The Fire Tetrahedron demonstrates the rapid, exothermic oxidation reaction that includes the uninhabited chain reaction.
Put simply, the Fire Tetrahedron is a slightly more up-to-date, scientifically grounded definition of the elements needed to ignite and sustain a fire. The only difference is the addition of an exothermic chain reaction to the traditional view. This additional fourth element aids in the ignition, the burning of a fire, and the continuation of combustion.
What is an exothermic reaction?
An exothermic chemical reaction is one that releases energy, generally through heat. These reactions can be halted using some more industry-specific fire extinguishers.
It is worth noting however, is that as of 2000 in Europe, and 2003 in the UK, the use of Halon or BCR fire extinguishers (to disrupt chain reactions) was banned except under exceptional circumstances due to it having the highest ozone- depleting capacity of any chemicals in use at the time! Halon and BCR extinguishers can now only generally be used in the military, aviation, vehicle, and fuel installations, and the Channel Tunnel.
Why use a model at all?
By using a pictorial model, we can understand how separating the elements involved can stop fires breaking out. We can also increase awareness on how fire extinguishers, and other fire-fighting products such as fire blankets, can remove one face of the tetrahedron (or one side of the triangle), allowing for the fire to be suppressed, controlled, or extinguished.
This geometric representation allows for greater fire awareness, improved fire confidence, and can save more lives being decimated from needless fires in the home and the workplace.
By understanding fire as a chemical reaction to a selection of elements, individuals have the capacity to stop seeing fire as a random chance event and understand how to prevent it. They can see the potential fuel, and ignition sources around them and keep them stored safely away from one another; thereby reducing the chance of fires breaking out. They can educate others who keep potential fuels near ignition sources and diminish the chance of future fires.
One illustration can bring about major change: fires can be prevented, and lives can be spared.
 Taken from HM Government, Regulatory Reform (fire safety) Order 2005, ‘A short guide to making your premises safe from fire,’ <https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/14879/making-your-premises-safe-short-guide.pdf>, page 8, 11.10.2021