What is the most common ignition source for flammable liquids?

Static electricity is recognized as the most common ignition source for flammable liquids due to its ability to generate sparks without requiring a significant amount of heat. When flammable liquids are present, even small discharges of static electricity can ignite vapors, leading to a fire or explosion. This is particularly prevalent in environments where liquids are being transferred, poured, or agitated, causing the potential for static buildup.

Mechanical heat sources, while they can ignite flammable liquids, typically require sustained heat or friction to create a flame and are not as prevalent as static discharge incidents. Nuclear energy and solar energy are not commonly associated with igniting flammable liquids, as they involve entirely different processes that do not apply to typical ignition scenarios in the context of fire safety. Thus, static electricity stands out as the primary ignition source in the context of flammable liquids.