Classification is the grouping of items into categories according to some consistent principle. The following paragraph classifies species of electric fish.

Scientists sort electric fishes into three categories. The first comprises the strongly electric species like the marine electric rays or the freshwater African electric catfish and South American electric eel. Known since the dawn of history, these deliver a punch strong enough to stun a human. In recent years, biologists have focused on a second category: weakly electric fish in the South American and African rivers that use tiny voltages for communication and navigation. The third group contains sharks, nonelectric rays, and catfish, which do not emit a field but possess sensors that enable them to detect the minute amounts of electricity that leak out of other organisms.

—Anne and Jack Rudloe, “Electric Warfare: The Fish That Kill with Thunderbolts”

Division takes one item and divides it into parts. As with classification, division should be made according to some consistent principle. The following paragraph describes the components that make up a baseball.

Like the game itself, a baseball is composed of many layers. One of the delicious joys of childhood is to take apart a baseball and examine the wonders within. You begin by removing the red cotton thread and peeling off the leather cover—which comes from the hide of a Holstein cow and has been tanned, cut, printed, and punched with holes. Beneath the cover is a thin layer of cotton string, followed by several hundred yards of woolen yarn, which makes up the bulk of the ball. Finally, in the middle is a rubber ball, or “pill,” which is a little smaller than a golf ball. Slice into the rubber and you’ll find the ball’s heart—a cork core. The cork is from Portugal, the rubber from southeast Asia, the covers are American, and the balls are assembled in Costa Rica.

—Dan Gutman, The Way Baseball Works