Traditional Culture Encyclopedia - Hotel franchise - What's the small one above the hotel room?

What's the small one above the hotel room?

The big one is a smoke detector, which will automatically alarm the hotel central fire control room when smoke is generated in the room. The small one circled is the temperature-sensitive self-explosion nozzle. After the fire, when the indoor temperature reaches a certain height, the glass head in front of this little thing (connected to the water pipe) will burst and spray water mist, automatically extinguishing the just-burned flame.