A fire occurs naturally when the elements are present and combined in the right mix, meaning that fire is actually an event rather than a thing. A fire can be prevented or extinguished by removing any of the elements of the fire triangle. For example, covering a fire with a fire blanket removes the oxygen part of the triangle and can extinguish a fire. In large fires where firefighters are called, decreasing the amount of oxygen is not usually an option because there is no effective way to make this happen over a prolonged period.