Diffusion is the movement of molecules from a region of high concentration to one of low concentration. If you have ever opened a bottle of cologne or perfume, you have witnessed diffusion.

Digestive System

The digestive system is a group of organs responsible for the conversion of food into nutrients and energy needed by the body. In humans, the digestive system consists of the mouth, esophagus, stomach, and small and large intestines.


Dinosaurs are a group of now-extinct, terrestrial reptiles in the order Dinosauria. They lived during the Mesozoic Era, from about 225 million years ago to 66 million years ago.


A diode is an electronic device that has two electrodes (conductors of electrical currents) arranged in such a way that electrons (subatomic particle having a negative charge) can flow in only one direction. Because of this ability to control the flow of electrons, a diode is commonly used as a rectifier—a device that converts alternating current into direct current.


The term dioxin refers to a large group of organic compounds that are structurally related to benzene (a colorless, flammable, and toxic [poisonous] liquid hydrocarbon, meaning it contains both carbon and hydrogen atoms) and may contain one or more chlorine atoms in their structures. Those compounds that do contain chlorine are known as chlorinated dioxins and are of the greatest environmental interest today.


Disease can be defined as any change in body processes that impairs its normal ability to function. The human body has certain basic requirements that must be met if it is to function normally.


Distillation is a technique by which two or more substances with different boiling points can be separated from each other. For example, fresh water can be obtained from seawater (water that contains salts) by distillation.

Doppler Effect

The Doppler effect is an effect observed in light and sound waves as they move toward or away from an observer. One simple example of the Doppler effect is the sound of an automobile horn.

Drift Net

Drift nets are free-floating nets used in oceans to snare fish by their gills. Each net can measure up to 50 feet (15 meters) deep and 55 miles (89 kilometers) long.


Drought is an extended period of exceptionally low precipitation. A drought can feature additional weather characteristics, including high temperatures and high winds.

DVD Technology

DVD stands for digital versatile disc, although it is also commonly referred to as digital video disc, due to the popularity of DVDs in the video industry. DVD technology allows for the storage of a large amount of data using digital technology.

Dyes and Pigments

Dyes and pigments are substances that impart color to a material. The term colorant is often used for both dyes (also called dyestuffs) and pigments.


Dyslexia is a learning disorder characterized by difficulty reading and writing. Dyslexia is not caused by poverty, psychological problems, lack of education, or laziness.


The human ear is the organ responsible for hearing and balance. The ear consists of three parts: the outer, middle, and inner ears.


Earth, the third planet from the Sun, is our home planet. Its surface is mostly water (about 70 percent) and it has a moderately dense nitrogen-and-oxygen atmosphere that supports life—the only known life in the universe.


An earthquake is an unpredictable event in which masses of rock shift below Earth's surface, releasing enormous amounts of energy and sending out shock waves that sometimes cause the ground to shake dramatically. Not all earthquakes are enormous, but they can become one of Earth's most destructive forces.

Earth Science

Earth science is the study of the physical components of Earth—its water, land, and air—and the processes that influence them. Earth science can be thought of as the study of the five physical spheres of Earth: atmosphere (gases), lithosphere (rock), pedosphere (soil and sediment), hydrosphere (liquid water), and cryosphere (ice).

Earth's Interior

The distance from Earth's surface to its center is about 3,975 miles (6,395 kilometers). Scientists have divided the interior of Earth into various layers, based on their composition.

Eating Disorders

Eating disorders are psychological conditions that involve either overeating, voluntary starvation, or both. The best-known eating disorders are probably anorexia nervosa, anorexic bulimia, and obesity.

Ebola Virus

The Ebola (pronounced ee-BO-luh) virus is the common name for a severe, often-fatal bleeding or hemorrhagic (pronounced hem-or-RAD-jik) fever that first appeared in 1976. It is caused by a new kind of virus called a filovirus (pronounced FY-low-vye-russ) that kills most of its victims with frightening speed.


In the animal kingdom, certain animals determine the location of an object by producing sounds, then interpreting the echoes that are created when those sounds bounce off the object. This process is called echolocation.


An eclipse refers to the complete or partial blocking of a celestial body by another body and can be used to describe a wide range of phenomena. Solar and lunar eclipses occur any time the Sun, the Moon, and Earth are all positioned in a straight line.


Ecology is the study of the relationships of organisms with their living and nonliving environment. No organism exists entirely independently of other living and nonliving things around it.


An ecosystem (or ecological system) is a collection of communities of organisms and the environment in which they live. Ecosystems can vary greatly in size.


Just about every solid material possesses some degree of elasticity, and so do most liquids. Some common highly elastic products are rubber bands, kitchen spatulas, and bicycle tires.

Electrical Conductivity

Electrical conductivity is the ability of a material to carry the flow of an electric current (a flow of electrons). Imagine that you attach the two ends of a battery to a bar of iron and a galvanometer.

Electric Arc

An electric arc is a device in which an electric current (a flow of electrons) is caused to flow between two points separated by a gas. The two points are called electrodes.

Electric Current

An electric current is usually thought of as a flow of electrons. When two ends of a battery are connected to each other by means of a metal wire, electrons flow out of one end (electrode or pole) of the battery, through the wire, and into the opposite end of the battery.