It's a hidden nozzle. There is a standard nozzle with a red glass tube on it. The working temperature is 68℃. However, in some high-end places, such as hotels, hidden nozzles are widely used, and you can only see the lower cover, which is connected with the upper end of the nozzle by low-temperature fusible alloy. Fusible alloy is solid below 68℃, but it will melt when the temperature is higher than 68℃, and the lower cover will fall off, exposing the hidden nozzle. When the sprinkler senses 68, it will explode to extinguish the fire.