Cigarette Smoke

Cigarette smoke contains cancer-causing substances called carcinogens. Cigarette smoking is the major cause of lung cancer and emphysema (a serious disease of the lungs).


A circle can be defined as a closed curved line on which every point is equally distant from a fixed point within it.

Circulatory System

The human circulatory system is responsible for delivering food, oxygen, and other needed substances to all cells in all parts of the body while taking away waste products. The circulatory system is also known as the cardiovascular system, from the Greek word kardia, meaning "heart," and the Latin vasculum, meaning "small vessel." The basic components of the cardiovascular system are the heart, the blood vessels, and the blood.

Clone and Cloning

A clone is a cell, group of cells, or organism produced by asexual reproduction that contains genetic information identical to that of the parent cell or organism. Asexual reproduction is the process by which a single parent cell divides to produce two new daughter cells.


Clouds are made up of minute water droplets or ice crystals that condense in the atmosphere. The creation of a cloud begins at ground level.


Coal is a naturally occurring combustible material consisting primarily of the element carbon. It also contains low percentages of solid, liquid, and gaseous hydrocarbons and/or other materials, such as compounds of nitrogen and sulfur.

Coast and Beach

The coast and beach, where the continents meet the ocean, are everchanging environments where the conflicting processes of erosion (wearing away) and sedimentation (building up) take place. Coast is the land that borders an ocean or large body of water.


Cocaine is a powerful drug that stimulates the body's central nervous system. Prepared from the leaves of the coca shrub that grows in South America, it increases the user's energy and alertness, reduces appetite and the need for sleep, and heightens feelings of pleasure.


Cockroaches are winged insects found in nearly every part of the world. Although they are one of the most primitive living insects, they are very adaptable and highly successful.


A coelacanth (pronounced SEE-luh-kanth) is a large, primitive fish found in the Indian Ocean. Described as a "living fossil" and once thought to be extinct, this deep-sea fish is believed to form one of the "missing links" in the evolution from fish to land animals.


Cognition is the act of knowing or the process involved in knowing. When we "know" something, it means that we are not only aware or conscious of it, but that we can, in a way, make some sort of judgment about it.


Colloids are mixtures whose particles are larger than the size of a molecule but smaller than particles that can be seen with the naked eye. Colloids are one of three major types of mixtures, the other two being solutions and suspensions.


Color is a property of light that depends on the frequency of light waves. Frequency is defined as the number of wave segments that pass a given point every second.


Combustion is the chemical term for a process known more commonly as burning. It is one of the earliest chemical changes noted by humans, due at least in part to the dramatic effects it has on materials.


A comet—a Greek word meaning "long-haired"—is best described as a dirty snowball. It is a clump of rocky material, dust, and frozen methane, ammonia, and water that streaks across the sky on a long, elliptical (ovalshaped) orbit around the Sun.

Compact Disc

A compact disc (CD), or optical disc, is a thin, circular wafer of clear plastic and metal measuring 4.75 inches (120 centimeters) in diameter with a small hole in its center. CDs store different kinds of data or information: sound, text, or pictures (both still and moving).

Complex Numbers

Complex numbers are numbers that consist of two parts, one real and one imaginary. An imaginary number is the square root of a real number, such as √−4;.

Composite Materials

A composite material (or just composite) is a mixture of two or more materials with properties superior to the materials of which it is made. Many common examples of composite materials can be found in the world around us.


Composting is the process of arranging and manipulating plant and animal materials so that they are gradually broken down, or decomposed, by soil bacteria and other organisms. The resulting decayed organic matter is a black, earthy-smelling, nutritious, spongy mixture called compost or humus.

Compound, Chemical

A chemical compound is a substance composed of two or more elements chemically combined with each other. Compounds are one of three general forms of matter.

Computer, Analog

A digital computer performs calculations based solely upon numbers or symbols. An analog computer, on the other hand, translates continuously changing quantities (such as temperature, pressure, weight, or speed) into corresponding voltages or gear movements.

Computer, Digital

The digital computer is a programmable electronic device that processes numbers and words accurately and at enormous speed. It comes in a variety of shapes and sizes, ranging from the familiar desktop microcomputer to the minicomputer, mainframe, and supercomputer.

Computer Software

Computer software is a package of specific instructions (a program) written in a defined order that tells a computer what to do and how to do it. It is the "brain" that tells the "body," or hardware, of a computer what to do.

Conservation Laws

Conservation laws are scientific statements that describe the amount of some quantity before and after a physical or chemical change.


A constellation is a group of stars that form a long-recognized pattern in the sky, as viewed from Earth. The stars that make up a constellation do not represent any meaningful order in the universe.

Continental Margin

The continental margin is that underwater plain connected to continents, separating them from the deep ocean floor. The continental margin is usually divided into three major sections: the continental shelf, the continental slope, and the continental rise.


Contraception, also called birth control, is the deliberate effort to halt conception a child (to keep a woman from becoming pregnant). Attempts to prevent pregnancy date back to ancient times and cultures.


Corals are a group of small, tropical marine animals that attach themselves to the seabed and form extensive reefs, commonly in shallow, warm-water seas. These reefs are made up of the calcium-carbonate (limestone) skeletons of dead coral animals.