Fire is the process of combustion of inflammable materials producing heat and light and (often) smoke. With all the comforts and luxuries we now have, we tend to forget just how important fire has been to humans historically. But even today, fire is used for cooking, heating homes and water, or other important processes like the generation of electricity. In order to make a fire, you would need a few basic things like wood, tinder, and preferably some flint. Having these items could potentially save your life if you were to get stranded somewhere for a long period of time.
When a flammable or combustible material combines with an oxidizer, like an oxygen gas or oxygen-rich compound, and is exposed to a some form of heat or temperature above flashpoint, a fire will start. This is usually referred to as the fire tetrahedron. Without all of the elements listed above, a fire will not start or continue burning. In other words, a flammable liquid will start to burn if the fuel and oxygen have just the right amount of proportions. Some of the fuel-oxygen mixes can require a catalyst, which is a substance that isn't involved directly with a chemical reaction during combustions.
Once the fire is started, there must by some sort of way to keep the fire going with "the further release of heat energy in the process of combustion and may propagate", just as long as the supply of oxidizers and fuel doesn't run out. A fire can be put out by taking away any of the the elements that is required for the fire tetrahedron. For example, think of a natural gas flame, like a stovetop burner. A fire can be put out by simply turning off the gas supply which removes the much needed fuel. Another few ways to kill a stovetop fire is to cover the flame completely, which uses up all of the oxidizing agents immediately, to pour water over the flame, which takes away all the heat sources faster that the fuel can produce it, and blowing really hard on a flame will also remove the heat quicker than the gas can make from the fuel source.
Making A Fire
There are lots of things that fires can be used for, but people generally take them for granted since we have such nice cookstoves and burners. If for some reason an accident happened where there was no way to eat anything unless it was cooked, the knowledge on how to build a fire would be essential. It would help in more ways than just cooking: it could be used to sterilize bandages or anything that had serious bacteria issues, dangerous animals tend to be scared of fires, flying bugs will not want to be near it, and if you are stuck in a situation that you can't get out of then a fire can turn into a signal.
It is very critical that the correct spot is chosen to build a fire. It should be a ventilated, but sheltered spot with minimal wind possible and has lots of burning supplies around it. Remove all burnable stuff away from the designated fire spot so that complete control is maintained and no ground fires start. Plus, if the fire is built over some rocks or on flat, solid ground, the likely hood that something will go wrong is cut down considerably.
In order for this fire to start, tinder is a must-have. Tinder is some sort of material that can light on just a spark if it is completely dry. Some examples are paper, leaves, grass, bark and resin. All of these will help start a fire more efficiently and quickly. The key to keeping the fire going is slowly "warming it up". It is not ideal to get a fire barely started and then throw a couple logs on it. 
Gather up some tinder and ball it up, but not too tight because there needs to be an air flow. Using some magnesium and a fling block, make a small pile of magnesium shavings on top of the tinder to help ignite the fire with the flint. Once the fire starts, hold the tinder up in the air and blow from underneath to add oxygen. This is just one way to start a fire. Another way is to use a magnifying glass, or piece of glass, on a sunny day, a 6-volt battery with some steel wool, some powder from bullets, or simple friction.
- How To Make A Fire Wilderness-Survival-Skills, 2003-2010
- How To Start A Fire Without Matches eHow contributing writer, published by eHow
- Chemistry of fire Wikipedia