Wet pipe sprinkler systems are the most common system in the world. In this system the sprinkler piping is constantly filled with water. When the temperature at the ceiling gets hot enough the glass bulb or fusible link in a sprinkler will break. Since the system is already filled with water, it is free to flow out of that sprinkler head. The temperature around that specific sprinkler head needs to be high enough to break the glass bulb or fusible link that is holding water back. Once that happens, water will immediately start flowing from only that head.