Do you love the great outdoors? Here in Wonderopolis we love to go hiking, kayaking, and camping. Our favorite thing to do in the outdoors is to start a big campfire and cook s'mores!

There's nothing quite like telling a few ghost stories by the light of a raging campfire. Its heat feels so good at the end of a long day when the temperatures start to drop. The smell of the smoke combined with the roasting of hot dogs makes our tummies tingle.

We also love the unique sound of a campfire. Sound might not be the first thing you think of when you think of a campfire, but burning wood definitely makes a distinct snap, crackle, and pop sound that many people find relaxing.

But what exactly is going on when a campfire makes noise? Why does burning wood crackle in a fire? The answer lies within the wood itself!

Before we look within the wood, let's first learn a bit about fire. Fire comes from a chemical reaction between oxygen in the air and a source of fuel, such as wood or gasoline. Luckily, most sources of fuel don't burst into flames spontaneously, since they're surrounded by oxygen all the time. Instead, fuel sources must be heated to their ignition temperature for the combustion reaction to occur.

For wood, the ignition temperature is around 300º F. At this temperature, the heat begins to decompose the wood's cellulose material. As this happens, the decomposing material takes several forms: volatile gases (smoke), char (pure carbon), and ash (unburnable minerals).

The actual burning of the wood occurs when the volatile gases reach temperatures around 500º F, at which point the molecules break apart and recombine with oxygen to form water, carbon dioxide, and other products. This is what we think of as the process of burning.

At the same time, the carbon in the char combines with oxygen in a much slower reaction. You're probably already familiar with char. You can buy it at the store, and it's called charcoal. Charcoal is simply wood that has been heated to the point where most of the volatile gases have been removed. That explains why charcoal burns with no smoke!

When you place a log of wood onto a campfire, it begins to burn. Inside the wood, there are tiny pockets of fluids, such as water and sap. As the wood burns, the fire heats these fluids as if they were in a pan on the stove.

The heat from the fire causes the fluids within wood to first boil and then vaporize into steam. This steam gets trapped in the pocket within the piece of wood. The trapped steam begins to exert pressure on the surrounding wood.

Eventually, the wood gives way. The snap, crackle, or pop sound you hear is the wood splitting along a crevice and releasing steam into the fire. If you've ever tried to use wet wood for firewood, you've probably noticed that it snaps, pops, and crackles much more than usual. That's because of the excess water trapped within the wood!

Wonder What's Next?

Tomorrow's Wonder of the Day is brand new…and really, really old!