A sprinkler is similar to a hose nozzle because it breaks up the stream of water into a fine spray. A cap seals the waterway. The cap is held in place by either a glass bulb or two thin pieces of metal that are soldered together.
A fire creates a a narrow plume of hot air and gasses that rise to the ceiling and spread out. When the hot gases reach the nearest sprinkler they will heat the fusible element that holds the cap in place. The cap will fall away and the sprinkler will spray water on the fire. Because the water immediately cools the hot fire gases, the other sprinklers won't open because there is not enough heat to melt their fusible element.
If the fire is hot enough that one sprinkler cannot handle it alone, hot gases will reach the next nearest sprinkler. Then that sprinkler would open to stop the fire. This design of opening only when there is enough heat limits the number of sprinklers to what is needed to stop the fire. Fire records show that 93 percent of fires were handled by only one sprinkler. In the remaining cases, two sprinklers handled an additional four percent. It took only three sprinklers to handle nearly all of the remaining 3 percent.
Keep in mind that these figures include large warehouses with high piles of combustible goods, some of them very combustible. In these cases, more than one sprinkler may be necessary to spray enough water to absorb the tremendous heat. In residential settings, the likelihood of more than one sprinkler opening is much more rare, and the number of fires controlled by one sprinkler is much closer to 100 percent.
The water spray from the sprinkler cools the fire gases over the fire. When the temperature of the burning material drops to below its combustion temperature, it can no longer burn and the fire goes out.
Responding firefighters will shut off the sprinkler only once they are sure that the fire is completely out.