Chris Chaney, age 11, of Rancho Santa Fe, Calif., for his question:

Is fire a solid, liquid or gas?

Solids, liquids and gases stay more or less as they are, at least until the temperature changes. But fire is a busy happening, always changing something into something else. It blazes with dancing flames, smolders with sooty smoke and gleams with glowing embers. And in all the busy burly burly, its solid or liquid fuel is changed into ashes and steamy gases.

Fire is a chemical reaction that can change solids, liquids or gases into different solids, liquids or gases. True, the solid logs in a blazing campfire seem to disappear. But actually the molecules in the solid fuel are changed into molecules of invisible gases. The few substances that refuse to burn at campfire temperatures are left behind as recognizable ashes.

Countless different chemical reactions go on all the time throughout the world of nature. All of them are busy rearranging atoms to create different molecule packages. In the process, some of these happenings use energy, while others give off energy. When a blazing fire rearranges the molecules of fuel, it gives off energy in the form of heat and light.

Actually the chemical story of your campfire began many long years ago. Growing trees used energy from sunlight to assemble atoms from dissolved chemicals, air and water to build woody molecules for their trunks. When the trees are cut down, logs from their trunks and branches form fuel for the campfire.

Two things are needed to start the fire. First, the fuel must be hot enough to start burning. So we raise the temperature by burning paper and sticks under our logs. However, the woody fuel cannot burn without the help of oxygen in the air. Fire is a chemical process called oxidation because it needs oxygen to make it go.

The trees used the energy of sunlight to bind their woody molecules together. In the fire, these molecules are broken apart, and the energy that bound them together is given off as light and heat. The fragments fly off as water vapor and carbon dioxide  and merge with the other invisible gases of the air.    

The fire itself is not a solid or a liquid or a gas. It is an energetic event which may involve all three of these forms of matter. In the case of the campfire, solid wood is transformed into gases and water vapor, smoky fragments of carbon plus a handful of solid ashes.     Slower forms of oxidation, believe it or not, go on inside the body. For example, digestion is a slow burning process that uses oxygen to convert the food we eat into substances that can be used by the busy cells.     Andy sends a 7 volume set of the Chronicles of Narnia to Teri Kendall, age 9, of Lawrence, Ind., for her question: What is an emu? The emu is a big bulky bird who belongs to Australia. He looks a lot like the African ostrich, though the ostrich has huge wings with feathery plumes. The emu has smallish wings, hidden among his thick silky feathers. A full size emu stands about 6 feet tall and weighs 100 pounds. Usually he strolls around, looking for plant food, green salads and fruits. But when he hurries, his long strong legs take him striding along at 30 m.p.h.