Oil Spills

Crude oil or petroleum is an important fossil fuel. Fossil fuels were formed millions of years ago, when much of Earth was covered by water containing billions of tiny plants and animals.


An orbit is the path a celestial object follows when moving under the control of another's gravity. This gravitational effect is evident throughout the universe: satellites orbit planets, planets orbit stars, stars orbit the cores of galaxies, and galaxies revolve in clusters.

Organic Chemistry

Organic chemistry is the study of compounds of carbon. The name organic goes back to a much earlier time in history when chemists thought that chemical compounds in living organisms were fundamentally different from those that occur in nonliving things.

Organic Farming

Organic farming is the process by which crops are raised using only natural methods to maintain soil fertility and to control pests. The amount of crops produced by conventional farming methods is often larger than that of organic farming.


Orthopedics is the branch of medicine that specializes in diseases of and injuries to bones. French physician Nicholas Andry coined the term "orthopedia" in his 1741 book on the prevention and correction of muscular and skeletal deformities in children.


Osmosis is the movement of a solvent, such as water, through a semi-permeable membrane. (A solvent is the major component of a solution, the liquid in which something else is dissolved.) A semipermeable membrane is a material that allows some materials to flow through it but not others.

Oxidation-Reduction Reaction

The term oxidation-reduction reaction actually refers to two chemical reactions that always occur at the same time: oxidation and reduction. Oxidation-reduction reactions are also referred to more simply as redox reactions.

Oxygen Family

The oxygen family consists of the elements that make up group 16 on the periodic table: oxygen, sulfur, selenium, tellurium, and polonium. These elements all have six electrons in their outermost energy level, accounting for some common chemical properties among them.


Ozone is an allotrope (a physically or chemically different form of the same substance) of oxygen with the chemical formula O3. This formula shows that each molecule of ozone consists of three atoms.


Paleoecology is the study of fossil organisms and their relationship to ancient environments. Paleoecology falls under the broader category of paleontology (the study of fossils).


Paleontology is the study of ancient life-forms of past geologic periods. Paleontologists learn about ancient animals and plants mainly through the study of fossils.


Paper is an indispensable part of everyday life. Beyond its use as the basic material for written and printed communication, paper in its various forms are used for hundreds of other purposes, including packaging, wrapping, insulating, and toweling.


A parasite is an organism that depends on another organism, known as a host, for food and shelter. As an example, tapeworms live in the digestive system of a large variety of animals.

Particle Accelerators

Particle accelerators (also known as atom-smashers) are devices used for increasing the velocity of subatomic particles such as protons, electrons, and positrons. Although they were originally invented for the purpose of studying the basic structure of matter, particle accelerators later found a number of practical applications.


Perception is the quality of being aware of the conditions in one's environment. For example, visual perception refers to the ability of an organism to see objects in the world around it.

Periodic Function

A periodic function is an event that occurs repeatedly in a very regular manner. Suppose that you stand on the beach near a lighthouse.

Periodic Table

The periodic table is a chart that shows the chemical elements and their relationship to each other. The periodic table is a graphic way of representing the periodic law.

Petroglyphs and Pictographs

Petroglyphs and pictographs are terms used by archaeologists to describe forms of rock carvings and paintings. Petroglyph refers to a rock carving or etching, while pictograph is commonly applied to a rock painting.


Petroleum, also called crude oil, is a thick, flammable, yellow-to-black colored liquid. Petroleum was first found oozing out of rocks on Earth's surface.


The most common method of indicating the acidity of a solution is by stating its pH. The term pH refers to a mathematical system developed by Danish chemist Søren Sørenson (1868–1939) around 1909.


A phobia is an abnormal or irrational fear of a situation or thing. A person suffering from a phobia may dwell on the object of his or her fear when it is not even present.


Photochemistry is the study of chemical changes made possible by light energy. The production of ozone in Earth's upper atmosphere is an example of such a change.


Photocopying is the process of photographically reproducing a document of text, illustrations, or other graphic matter. The most common photocopying method used today is called xerography (from the Greek words for "dry" and "writing").

Photoelectric Effect

When visible light, X rays, gamma rays, or other forms of electromagnetic radiation are shined on certain kinds of matter, electrons are ejected. That phenomenon is known as the photoelectric effect.


Photosynthesis is the process by which green plants and certain types of bacteria make carbohydrates, beginning only with carbon dioxide (CO2) and water (H2O). Carbohydrates are complex chemical compounds that occur widely in plants and that serve as an important food source for animals.


Phototropism (pronounced foe-TA-tro-piz-em) is the growth of a plant in the direction of its light source. Plants are very sensitive to their environment and have evolved many forms of "tropisms" in order to ensure their survival.

Physical Therapy

Physical therapy is the use of exercise, heat, cold, water, massage, or electricity in the treatment of damaged muscles, bones, or joints due to injury or disease. The goal of physical therapy is to restore full or partial function of the affected body part or to build up other muscles to make up for weak ones.


Physics is the science that deals with matter and energy and with the interaction between them. Perhaps you would like to determine how best to aim a rifle in order to hit a target with a bullet.