The abacus is an ancient calculating machine. This simple apparatus is thought to have originated in Babylon about 5,000 years ago.


Abrasives are materials used to wear down, smooth, clean, shape, or polish a surface. The perfectly smooth surfaces needed in telescope mirrors, for example, are produced by grinding a piece of glass or metal with abrasives.


Acceleration is a measure of the rate at which the velocity of an object is changing. If you are riding in a car traveling in a straight line at a constant 50 kilometers per hour, you experience no acceleration because the car's velocity (rate of motion) is not changing.

Acetylsalicylic Acid

Acetylsalicylic (pronounced uh-SEE-tuhl-sa-luh-si-lik) acid, commonly known as aspirin, is the most popular therapeutic drug in the world. Sold without a prescription as tablets, capsules, powders, or suppositories (a meltable form for insertion into a body cavity), it reduces pain, fever, and swelling, is believed to decrease the risk of heart attacks and strokes, and may protect against colon cancer and help prevent premature birth.

Acid Rain

Acid rain is a popular phrase used to describe rain, snow, fog, or other precipitation that is full of acids that collect in the atmosphere due to the burning of fuels such as coal, petroleum, and gasoline. Acid rain was first recognized in Europe in the late 1800s but did not come to widespread public attention until about 1970, when its harmful effects on the environment were publicized.

Acids and Bases

Acids and bases are chemical compounds that have distinctive properties in water solution. The sour taste of a lemon, lime, or grapefruit, for example, is caused by citric acid.


Acoustics is the science that deals with the production, transmission, and reception of sound. The first scientist to study sound scientifically was German physicist Ernst Florens Friedrich Chladni (1756–1827).


The actinides (or actinoids) are the chemical elements with atomic numbers between 90 and 109 inclusively. (An atomic number indicates the number of protons in an atom.) Actinides occur between Groups 3 and 4 in Period 7 of the periodic table.


Adaptation is a term used to describe the ways in which organisms change over time in response to the changing demands of their environment. Organisms seem to accumulate certain physiological, behavioral, and structural traits gradually, and these traits aid them in their ability to survive and reproduce under existing environmental conditions.


Addictions can be physical (of the body), psychological (of the mind), or both. In fact, almost any behavior can be termed an addiction if it becomes the primary focus of a person's life, and especially if it results in harmful effects to one's physical health and well-being.


Adhesives bond two or more materials at their surfaces. Any time you reattach a handle that has broken off a cup, paste a photograph into an album, or tape a message to a friend's door, you use an adhesive.


Aerodynamics is the study of air flow over airplanes, cars, and other objects. Airplanes fly because of the way in which air flows over their wings and around their bodies, so a knowledge of aerodynamics is crucial to the design and construction of airplanes.


Aerosols are collections of tiny particles of solid and/or liquid suspended in a gas. The size of particles in an aerosol ranges from about 0.001 to about 100 microns.


Africa is the world's second largest continent, encompassing an area of 11,677,240 square miles (30,244,051 square kilometers), including offshore islands. Recognized as the birthplace of the human race and of many other animal and plant species, it also possesses the world's richest and most concentrated deposits of minerals such as gold, diamonds, uranium, chromium, cobalt, and platinum.

Agent Orange

Agent Orange is one of several herbicidal (plant-killing) preparations that was used by the U.S. military to destroy forests and enemy crops in Vietnam in the 1960s.

Aging and Death

Aging is a series of biological changes that follow a natural progression from birth through maturity to old age and death. For most people, advancing age is characterized by graying or thinning hair, loss of height, wrinkling of the skin, and decreased muscular strength.


The development of agriculture—the raising of crops and animals for food—has been fundamental to the development of civilization. Farming brought about the settlement of farm communities, which grew into towns and city-states.


An agrochemical is any substance used to help manage an agricultural ecosystem, or the community of organisms in a farming area. Agrochemicals include: (1) fertilizers, (2) liming and acidifying agents, (3) soil conditioners, (4) pesticides, and (5) chemicals used in animal husbandry, such as antibiotics and hormones.

AIDS (Acquired Immunodeficiency Syndrome)

Acquired immunodeficiency syndrome (AIDS) is a disease caused by the human immunodeficiency virus (HIV). HIV damages the immune system by attacking certain white blood cells called lymphocytes (specifically those called helper T cells).


An aircraft is a machine used for traveling through the atmosphere. There are two types of aircraft: the first type are known as lighter-than-air crafts, which are supported by their own buoyancy or ability to float.

Air Masses and Fronts

An air mass is a large body of air that, at any one height, has a relatively steady temperature and moisture content throughout. Air masses typically cover areas ranging from hundreds of thousands to millions of square miles.


Alchemy was an early system of thinking about nature that contributed to the development of the modern science of chemistry. It was popular in ancient China, Persia, and western Europe throughout antiquity and the Middle Ages (400–1450).


Alcoholism is a serious, chronic (can be curbed or regulated but cannot be cured), potentially fatal condition in which a person has an uncontrollable urge to drink alcoholic beverages. Alcoholism can be seen in both the rich and the poor, the young and the old, and males and females.


Alcohols are organic (carbon-containing) compounds derived from hydrocarbons (organic compounds composed of only carbon and hydrogen). They are formed by replacing the hydrogen atoms in their parent hydrocarbons with one or more hydroxyl groups.


Algae (singular: alga) are plants or plantlike organisms that contain chlorophyll (pronounced KLOR-uh-fill) and other pigments (coloring matter) that trap light from the Sun. This light energy is then converted into food molecules in a process called photosynthesis.


Algebra is often referred to as a generalization of arithmetic: problems and operations are expressed in terms of variables as well as constants. A constant is some number that always has the same value, such as 3 or 14.89.

Alkali Metals

The alkali metals are the elements that make up Group 1 of the periodic table: lithium, sodium, potassium, rubidium, cesium, and francium. These elements are all much too active to exist in nature in their native state.

Alkaline Earth Metals

The elements that make up Group 2 of the periodic table are commonly called the alkaline earth metals. They include beryllium, magnesium, calcium, strontium, barium, and radium.