Today’s Wonder of the Day was inspired by Mariano. Mariano Wonders, “How do fire extinguishers work?” Thanks for WONDERing with us, Mariano!

Can you imagine what it would've been like to be one of the first prehistoric people to roam Earth? Figuring out how to navigate the world around you and accumulate the basic food, water, and shelter you'd need to survive probably would've consumed most of your time.

Depending upon which part of Earth you inhabited, there might have been another resource you'd need to survive: fire. With the exception of tropical areas, most other areas of the world experience extended seasons of cold weather that would've been hard to survive without fire.

Whether you harnessed fire from a tree struck by lightning during a storm or learned how to make it with flint and kindling, you would probably quickly learn that it had benefits beyond just keeping you warm at night. For example, roasting a rabbit over a fire must have seemed like a delicacy compared to raw meat.

Even the earliest prehistoric peoples had to learn how to contain fire to use it safely. An unchecked fire could quickly spread to nearby trees and turn a lush forest into an uninhabitable wasteland in a matter of hours.

Likewise, modern humans must know how to use fire responsibly. Fire is great when it's burning the wick of a candle or heating up a skillet of bacon. It's not good when it's consuming your curtains or spreading across your carpet and climbing your walls.

Accidents certainly happen from time to time, especially when fire is involved. To prevent fires from spreading and destroying everything in their path, most buildings and homes contain one or more devices that can be used to put out a fire in a hurry: fire extinguishers.

Before you can understand how fire extinguishers work, it's helpful to understand how fire itself works. Fire results from a chemical combustion reaction that requires three things: intense heat, oxygen, and fuel, such as wood or gasoline.

Fire extinguishers work by removing one or more of those three necessary elements of fire. This usually occurs by applying a substance that either cools the burning fuel or displaces the oxygen surrounding the fuel.

Most fire extinguishers consist of a metal cylinder filled with either water or a dry chemical foam or powder that acts as a smothering agent. They work much like an aerosol can. When you depress a lever, a propellant expels the material inside under high pressure.

When using a fire extinguisher, you should always direct the spray of water, foam, or powder toward the fuel source, not the flames. If you can cool the fuel source or cut off its supply of oxygen, the fire will die out.

Fire extinguishers are best used for small fires that can be easily contained. This is because most fire extinguishers contain only a limited amount of water, foam, or powder, which can be used up in seconds when it's expelled at high pressure. Larger fires require bigger equipment and the professionals that know how to use it.

One of the most common and effective types of fire extinguisher is the water extinguisher. Water quickly cools fuel sources such as wood, paper, and cardboard. Water is not a good choice for all fires, however.

Electrical fires and grease fires, for example, can be made much worse by dumping water on them. Water can conduct a current in the case of an electrical fire, leading to the danger of electrocution. Since oil and water don't mix, pouring water on a grease fire will spread the hot grease everywhere while the water simply evaporates.

Electrical fires and fires involving inflammable liquids should be put out with a different type of extinguisher, such as a carbon dioxide extinguisher or a dry chemical foam or powder extinguisher. These extinguishers contain smothering agents, like pure carbon dioxide, sodium bicarbonate (baking soda), potassium bicarbonate, or monoammonium phosphate.

These materials smother the fire by displacing the oxygen with carbon dioxide. In the case of carbon dioxide extinguishers, the extinguisher releases pure carbon dioxide gas directly. Dry chemical foams or powders decompose when they hit the fire, releasing carbon dioxide in the process.

Wonder What's Next?

Tomorrow’s Wonder of the Day may make you want to wash your hands…or not!