What is the most common fire extinguishing agent?

Water is regarded as the most common fire extinguishing agent due to its accessibility, effectiveness, and versatility in combating various types of fires. It works primarily by cooling the burning material below its ignition temperature and by smothering the fire by eliminating oxygen. Water can be used effectively on fires involving solid combustibles, such as wood, paper, and textiles, making it particularly suitable for structural fires that firefighters frequently encounter.

While other agents like foam, dry chemicals, and carbon dioxide have specific applications and benefits—such as foam's effectiveness on liquid fires or dry chemical's capacity to disrupt chemical reactions—water remains the fundamental tool in fire suppression due to its widespread availability and ability to handle a variety of fire scenarios effectively. This is why water is considered the most common fire extinguishing agent.