The glass bulb contains a specially formulated glycerin-based liquid. When a fire occurs, the rising air temperature heats the bulb. The liquid expands rapidly until it exceeds the structural strength of the glass, causing the bulb to shatter. Once the glass breaks, the water pressure pushes the sealing plug out, and water discharges against the deflector, creating a uniform spray. The standard red bulb shatters at 68°C (155°F).