Laura Kelley, aged 15, of Ward, W. Va. or her question:

How does coal burn?

Coal, like everything else, is made of molecules. The molecules are bundles of atoms bound together with energy. When it burns, the coal goes through a chemical change. Its molecules break apart. The coal is changed into other substances, most of them invisible The energy that bound the atoms together in molecules goes off as heat.

Lets see how the coal molecules were built in the first place. This too, was a chemical change„ It was a cementing job and it used up energy instead of giving off energy. The energy, of course; was sunlight as it fell upon ancient forests.

The trees used the sunlight in their magic recipe for making plant food. Some of the food was rebuilt into cellulose, the stuff that makes wood. Cellulose molecules are made of carbon$ hydrogen and oxygen. The plants took these atoms from the air and from moisture. They used the energy of sunlight to tie them up in complicated molecules.

Molecules are not packed close together in a solid‑looking log of wood. Actually, they don't touch each other. Most of a molecule, most of and atom, is empty space. What's more, the molecules in the solid wood are always on the move. Even the atoms inside the molecules move, and so do the particles of each tiny atom. In time, this busy wood was buried and pressed into coal. Its molecules were pressed closer together and became less active.

At last a lump of the coal arrives in a grate. It feels the blazing kindling below it, The heat stirs its lazy molecules into action. This, in turn makes the coal hot. For heat is merely fast‑moving molecules. When it reaches ignition point, the coal starts to burn. Because of the wag it is built, the coal needs a higher ignition temperature than does wood or paper.

At ignition point, the molecules of coal are whirling fast enough to fly apart. The bundles come undone. They break into the original atoms used by the tree to make molecules of wood. The hems energy released is sunlight which has sunlight which had been bottled up for millions of years.

Some of the carbon atoms rush off to join with oxygen in the air: One carbon atom and two atoms of Oxygen team up to make a brand new molecule of carbon dioxide gas. Hydrogen atoms team up with atoms of oxygen to form molecules of water vapor. Some of the carbon swirls off as smoke and soot. Most of the solid wood becomes invisible gases,

These gases fly off at great speed. They reach their ignition point and burst into flames. Soon only ashes are left. These are the few substances in the coal that will not burn at the temperature in the grate.