The pipes for fire sprinkler systems contain highly pressurized water. Once the mechanism in the fire sprinkler head has been triggered, the water will only be able to be turned off by an emergency responder.
Each sprinkler head has a little plug to hold back the pressurized water in the pipes. However, when the temperature rises above 165 degrees Fahrenheit, the plug is released. How the plug is released depends on the type of fire sprinkler system installed.
Some systems have a special solder for the sprinkler head that melts in high temperatures. Other systems may have a glass vial that contains fluid that will expand to break the glass in high temperatures.