Contrary to popular belief, fire sprinklers are not trigged by smoke. Fire sprinklers work because high heat triggers the sprinkler system. When a blaze ignites, the air directly above it heats rapidly. This hot air rises and spreads along the ceiling. When the air is hot enough and reaches a sprinkler head, it triggers a chain reaction. Most sprinkler heads feature a glass bulb filled with a glycerin-based liquid. This liquid expands when it comes in contact with air heated above 57 degrees Celsius. When the liquid expands, it shatters its glass confines and the sprinkler head activates. They can also be triggered if someone breaks a glass bulb by accident.