Explosives are materials that produce violent chemical or nuclear reactions. These reactions generate large amounts of heat and gas in a fraction of a second.

Extrasolar Planet

Extrasolar planets, or exoplanets, are planets that exist outside our solar system. These planets may orbit stars other than our Sun or move independently through interstellar space.


The eye is the organ of sight (vision) in humans and animals. The eye works by transforming light waves into visual images.


A fault is a crack or fracture in Earth's crust caused by the movement of landmasses, called plates, on either side of the fault line. Faults are found either at the surface (fault surface) or underground (fault plane).


Felines, generally known as cats, are mammals in the family Felidae of the order Carnivora. Cats are the most carnivorous of all meat-eating animals.


In its broadest sense, fermentation refers to any process by which large organic molecules are broken down to simpler molecules as the result of the action of microorganisms (organisms so small they can be seen only with the aid of a microscope). The most familiar type of fermentation is the process by which sugars and starches are converted to alcohol by enzymes in yeasts.


Fertilization is the process by which the nucleus of a sperm (a male reproductive cell) fuses (combines) with the nucleus of an egg (a female reproductive cell; also called an ovum). Fertilization occurs somewhat differently in plants and animals.

Fiber Optics

Optical fiber is a very thin strand of glass or plastic capable of transmitting light from one point to another. Since the late 1950s, optical fibers have emerged as revolutionary tools in the fields of medicine and telecommunications.


Filtration is the process by which solid materials are removed from a fluid mixture, either a gas or liquid mixture. Anyone who has ever prepared foods in a kitchen has probably seen one of the simplest forms of filtration.


Ocean saltwater covers more than three-quarters of Earth's surface; lakes, rivers, streams, ponds, canals, swamps, marshes, and other forms of freshwater cover vast expanses of the planet's surface as well. One of the most successful groups of animals that have evolved to fill all these habitats are fish.


A flower is the reproductive part of a plant that produces seeds. Plants that produce flowers and fruit are called angiosperms.

Fluid Dynamics

Fluid dynamics is the study of the flow of liquids and gases, usually in and around solid surfaces. For example, fluid dynamics can be used to analyze the flow of air over an airplane wing or over the surface of an automobile.

Fluorescent Light

Fluorescent light is the most common type of electrical light used in the United States. It is estimated that there are 1.5 billion fluorescent lamps in use nationwide, mostly in commercial settings such offices, factories, stores, and schools.


Fluoridation is the process of adding the chemical fluoride to a substance (often drinking water) to reduce tooth decay. In the human body, fluoride acts to prevent tooth decay by strengthening tooth enamel and inhibiting the growth of plaque-forming bacteria.

Food Preservation

Food preservation refers to any one of a number of techniques used to prevent food from spoiling. All foods begin to spoil as soon as they are harvested or slaughtered.

Food Web and Food Chain

The terms food chain and food web both refer to groups of organisms that are dependent on each other for food. A food chain is a single series of organisms in which each plant or animal depends on the organism above or below it.

Forensic Science

Forensic science is the application of science to matters of law. A basic principle of forensic science is that a criminal always brings something to the scene of a crime that he or she leaves behind.


Forestry is usually defined as the science of the harvesting, planting, and tending of trees, primarily in managed forested landscapes. In the first 250 years after Europeans came to North America, little or no effort was made to protect the continent's forest resources.


A forest is an ecosystem or ecological community whose most important organisms are trees. Forests occur any place where the climate provides a sufficiently long growing season, adequate air and soil temperature, and suitable amount of moisture.

Formula, Chemical

A chemical formula is a combination of chemical symbols that represents the chemical composition of a compound. At a minimum, a formula tells which elements are present in the compound and the relative amount of each element.

Fossil and Fossilization

A fossil is the remains or traces of a once-living plant or animal that was preserved in rock or other material before the beginning of recorded history. The term also is used to describe the fossil fuels (oil, coal, petroleum, and natural gas) that have been formed from the remains of ancient plants and animals.


A fractal is a geometric figure with two special properties. First, it is irregular, fractured, fragmented, or loosely connected in appearance.

Fraction, Common

A fraction is a mathematical expression that states the ratio between two numbers. The adjective common in the term common fraction means that both numbers involved are integers, or whole numbers.


Any process that repeats on a regular basis has an associated frequency. The frequency is the number of repetitions, or cycles, that occur during a given time interval.


Friction is a force that resists motion when the surface of one object slides over the surface of another. Frictional forces are always parallel to the surfaces in contact, and they oppose any motion or attempted motion.


A function is a mathematical relationship between two sets of real numbers. These sets of numbers are related to each other by a rule that assigns each value from one set to exactly one value in the other set.


Fungi (plural of fungus) are one of the five kingdoms of organisms. Kingdoms are the main divisions into which scientists classify all living things on Earth.

Gaia Hypothesis

The Gaia (pronounced GAY-ah) hypothesis is the idea that Earth is a living organism and can regulate its own environment. This idea argues that Earth is able to maintain conditions that are favorable for life to survive on it, and that it is the living things on Earth that give the planet this ability.