Ionization is the process in which one or more electrons are removed from an atom or molecule. The charged particle that results is called an ion.


An island is a relatively small area of land that is completely surrounded by water. It is impossible to give a total number to the islands that exist on the surface of the planet.


Isotopes are two forms of an element with the same atomic number but different mass number. The existence of isotopes can be understood by reviewing the structure of atoms.

Jet Engine

A jet engine is a heat engine that is propelled in a forward direction as the result of the escape of hot gases from the rear of the engine. In an airbreathing jet engine, air entering the front of the engine is used to burn a fuel within the engine, producing the hot gases needed for propulsion (forward movement).


Jupiter, the fifth planet from the Sun, is the largest and most massive planet in our solar system. It is 1,300 times larger than Earth, with more than 300 times the mass of Earth and 2.5 times the mass of all the other planets combined.

Kangaroos and Wallabies

Kangaroos and wallabies are pouched mammals, or marsupials, of Australia and nearby islands that are famous for their great leaping ability. The name kangaroo is usually used for large species and wallaby for smaller ones.


Lakes are inland bodies of standing water. Although millions of lakes are scattered over Earth's surface, most are located in higher latitudes and mountainous areas.


The lanthanides are the chemical elements found in Row 6 of the periodic table between Groups 3 and 4. They follow lanthanum (La), element #57, which accounts for their family name.


A laser is a device used to create a narrow, intense beam of very bright light. Laser stands for Light Amplification by Stimulated Emission of Radiation.

Laws of Motion

The term laws of motion generally refers to three statements originally devised by English physicist Isaac Newton (1642–1727) in the 1680s. These laws, along with Newton's law of gravitation, are generally considered to be the ultimate solution to a problem that had troubled scholars for more than 2,000 years: motion.


A leaf is a plant's principal organ of photosynthesis, the process by which sunlight is used to form foods from carbon dioxide and water. Leaves also help in the process of transpiration, or the loss of water vapor from a plant.

LED (Light-Emitting Diode)

An LED, or light-emitting diode, is a special type of diode that emits light when connected in a circuit. A diode is an electronic device that has two electrodes arranged in such a manner as to allow electrical current to flow in one direction only.

Legionnaires' Disease

Legionnaires' disease is an acute respiratory infection caused by a common bacteria that results in a serious case of pneumonia. It first became a well-known disease in 1976 when a serious outbreak occurred among a large number of people attending an American Legion convention.


Lenses are carefully shaped pieces of glass, plastic, or other transparent material. They are designed to manipulate light rays to create particular kinds of images.


Light is generally defined as that portion of the electromagnetic spectrum with wavelengths between 400 and 700 nanometers (billionths of a meter). Like all forms of electromagnetic radiation, light travels with a speed of 186,282 miles (299,728 kilometers) per second in a vacuum.


The speed of light is one of the most fundamental measurements in astronomy. Measured in miles or kilometers per second, the speed of light determines distance.


The lipids are a class of biochemical compounds, many of which occur naturally in plants and animals. (Biochemical compounds are organic compounds that are intimately involved in living organisms.) Most organic compounds are classified into one of a few dozen families, based on their structural similarities.


A lock or water lock is an enclosed, rectangular chamber with gates at each end, within which water is raised or lowered to allow boats or ships to overcome differences in water level. Locks have a history of over 2,000 years, and although they are most often used by boats on canals, they also are used to transport massive ships between seas.


In the 1500s and early 1600s, although science, engineering, and medicine were flourishing, many people did not understand multiplication tables. Mathematicians, astronomers, navigators, and scientists were forced to spend a lot of time performing calculations, so that little time was left to work on experiments and new discoveries.


The term luminescence is used to describe a process by which light is produced other than by heating. The production of light from heat, or incandescence, is familiar to everyone.

Lymphatic System

The lymphatic system is a network of vessels that transports nutrients to the cells and collects their waste products. The lymph system consists of lymph capillaries and lymph vessels that are somewhat similar to blood capillaries and blood vessels.

Machines, Simple

A simple machine is a device for doing work that has only one part. Simple machines redirect or change the size of forces, allowing people to do work with less muscle effort and greater speed, thus making their work easier.

Magnetic Recording/Audiocassette

Audiocassette tape recorders are widely used to record and play back music or speech. Information is stored on a narrow ribbon of plastic tape that has one side coated with a magnetic material consisting of magnetically active particles, most commonly iron oxide and chromium dioxide.


Magnetism is a fundamental force of nature manifested by the attraction of certain materials for iron. Materials so attracted are said to be magnetic materials.


Malnutrition is a condition in which a person's diet is inadequate to meet minimum daily requirements for nutrients such as proteins, fats, vitamins, and minerals. It is caused by one of two factors.


More than 4,000 species of living mammals belong to the vertebrate class Mammalia. This diverse group of animals has certain common features: all have four legs, bodies covered by hair, a high and constant body temperature, a muscular diaphragm used in breathing, a lower jaw consisting of a single bone, and three bones in the middle ear.


Marijuana is the common name for the drug obtained from the hemp plant, Cannabis sativa. Hemp is a tall annual plant that can grow in almost any climate.


Mars, the fourth planet from the Sun, was named for the Roman god of war. It is a barren, desolate, crater-covered world prone to frequent, violent dust storms.