The operating mechanism is a made up of glass bulb which contains a heat responsive liquid, inside the heat responsive liquid it contain a small bubble. During a fire, the ambient temperature rises causing the Bubble in the liquid to expand. 

When the ambient temperature reaches the rated temperature of the sprinkler, the bulb shatters which cause the water in the sprinkler system to flow through the deflector and create an umbrella spray pattern to kill the fire. 

Each sprinkler is activated individually when it is heated to its design temperature. See the table below: