Rockets and Missiles

The term rocket is used in two different ways: (1) it is used to describe a jet engine that does not depend on air for its operation but instead carries its own fuel and oxidizing agent, and (2) it is used to describe any vehicle that is powered by a rocket engine.


Rocks are composed of minerals, which are natural inorganic (nonliving) substances with specific chemical compositions and structures. A rock may consist of many crystals of one mineral or combinations of many minerals.


In astronomy, the word satellite refers to any single object that is orbiting another larger, more massive object under the influence of their mutual gravitational force.


Saturn, the sixth planet from the Sun, is named for the Roman god of agriculture, who was based on the Greek god Cronus. The second largest planet in the solar system, it measures almost 75,000 miles (120,600 kilometers) in diameter at its equator.


Savant is a name used to describe a person who has extraordinary skills in a very specialized area, but who is nonetheless intellectually disabled. It properly describes a rare phenomenon or syndrome in which a person with a severe mental handicap displays genius-like ability in a narrow field or area.


Schizophrenia (pronounced skiht-zo-FREH-nee-uh) is a severe mental condition that interferes with normal thought processes, causing delusions, hallucinations, and mental disorganization. As the most common of the extremely serious mental disorders known as psychosis (pronounced sy-KO-sis), it affects men and women equally, is found all over the world, and is usually a long-term illness with no definite cure.

Scientific Method

The term scientific method refers in general to the procedures that scientists follow in obtaining true statements about the natural world. As it happens, scientists actually use all manner of procedures to obtain the information they want.


Seasons on Earth are characterized by differences in temperature and the length of daylight. The four distinct seasons—spring, summer, autumn (or fall), and winter—are found only in the temperate zones.


A seed is a part of a flowering plant involved in reproduction. It consists of three major parts: the embryo, endosperm, and testa.

Set Theory

A set is a collection of things. A set can consist of real or literal numbers (such as 1, 2, 3, 4 or a, b, c, d) or of objects (such as baseballs or books).

Sexually Transmitted Diseases

Long known as venereal disease, after Venus, the Roman goddess of love, sexually transmitted diseases (STDs) are among the most common infectious diseases in the world. (An infectious disease is one caused by a parasite that attacks a host and begins to multiply, interfering with the normal life functions of the host.) There are more than 20 known STDs, ranging from life-threatening to painful and unsightly.

Skeletal System

Inside every person is a skeleton, a sturdy framework of 206 bones. The skeleton protects the body's organs, supports the body, and provides attachment points for muscles to enable body movement.

Slash-and-Burn Agriculture

Slash-and-burn agriculture refers to the process of cutting down a forest, burning the trees, and then using the cleared land to grow crops. This agricultural approach—used mainly in tropical countries—is the leading cause of tropical deforestation.

Sleep and Sleep Disorders

Sleep is a normal state of decreased consciousness and lowered metabolism during which the body rests. As a natural, necessary, and daily experience for humans and most other vertebrates (animals that have a backbone or spinal column), sleep has four stages through which we cycle several times a night.


Smell, called olfaction, is the ability of an organism to sense and identify a substance by detecting tiny amounts of the substance that evaporate and produce an odor. Smell is the most important sense for most organisms.


Snakes are limbless reptiles with long, cylindrical bodies, scaly skin, lidless eyes, and a forked tongue. Most species are not poisonous, some are mildly poisonous, and others produce a deadly poison.

Soaps and Detergents

A detergent is a cleaning agent. Detergents can be classified into one of two general categories: natural soaps (or just soaps) and synthetic detergents (or syndets).


Soil, which covers most of the land surface of Earth, is a complex mixture of weathered rock debris and partially decayed organic (plant and animal) matter. Soil not only supports a huge number of organisms below its surface—bacteria, fungi, worms, insects, and small mammals—but it is essential to all life on the planet.

Solar System

Our solar system consists of the Sun and all of its orbiting objects. These objects include the planets with their rings and moons, asteroids, comets, meteors and meteorites, and particles of dust and debris.


A solution is a homogeneous mixture of two or more substances. The term homogeneous means "the same throughout." For example, suppose that you make a solution of sugar in water.


Sonar, an acronym for sound navigation and ranging, is a system that uses sound waves to detect and locate objects underwater.

South America

South America, the fourth largest continent on Earth, encompasses an area of 6,880,706 square miles (17,821,028 square kilometers). This is almost 12 percent of the surface area of Earth.


The term space has two general meanings. First, it refers to the three-dimensional extension in which all things exist and move.

Spacecraft, Manned

Since 1961, hundreds of men and women from more than a dozen countries have traveled in space. Until the 1980s, however, most of those people came from the United States and the former Soviet Union.

Space Probe

A space probe is any unmanned spacecraft designed to carry out physical studies of the Moon, other planets, or outer space. Space probes take pictures, measure atmospheric conditions, and collect soil samples then bring or report the data back to Earth.

Space Station, International

The International Space Station (ISS) is a permanent Earth-orbiting laboratory that will allow humans to perform long-term research in outer space. Led by the United States, the ISS draws upon the scientific and technological resources of sixteen nations.


Spectroscopy is a science concerned with the analysis of the composition of matter based on the kind of radiation emitted by that matter. For example, suppose that a piece of iron is heated until it begins to glow.


The term spectrum has two different, but closely related, meanings. In general, the term refers to a whole range of things.