Mary Martinell, age 12, of San Diego, California, for her question:

What exactly is smoke?

The smoke from a campfire is a floating cloud of sooty fragments of carbon. The solid fragments are small enough to hang suspended in the air and the hot rising gases from the fire carry them aloft in smoky grey puffs and plumes. The various smokes that belch from factories and foundries may be mixed with a multitude of gaseous fumes and industrial wastes. The content depends on the fuels and other chemicals involved in the operations.

Fire is a chemical reaction that uses oxygen to break apart molecules of fuel. Fires that puff ordinary grey smoke are fed by such fuels at coal, wood and paper. These are hydrocarbon chemicals originally made by the plant world. The main ingredients in their molecule packages are atoms of hydrogen and carbon. The heat of the fire loosens their bonds and frees their basic ingredients. Freed hydrogen atoms join the gases of the air, and many combine with oxygen or carbon to form new molecules. Other carbon atoms cling together in tiny sooty fragments and plume forth in puffs of smoke.