## Mass

One common method of defining mass is to say that it is the quantity of matter an object possesses. For example, a small rock has a fixed, unchanging quantity of matter.

## Mass Production

Mass production is the manufacture of goods in large quantities using standardized designs so the goods are all the same. Assembly-line techniques are usually used.

## Mass Spectrometry

Mass spectrometry is a method for finding out the mass of particles contained in a sample and, thereby, for identifying what those particles are. A typical application of mass spectrometry is the identification of small amounts of materials found at a crime scene.

## Mathematics

Mathematics is the science that deals with the measurement, properties, and relationships of quantities, as expressed in either numbers or symbols. For example, a farmer might decide to fence in a field and plant oats there.

## Matter, States of

Matter is anything that has mass and takes up space. The term refers to all real objects in the natural world, such as marbles, rocks, ice crystals, oxygen gas, water, hair, and cabbage.

## Mendelian Laws of Inheritance

Mendelian laws of inheritance are statements about the way certain characteristics are transmitted from one generation to another in an organism. The laws were derived by the Austrian monk Gregor Mendel (1822–1884) based on experiments he conducted in the period from about 1857 to 1865.

## Mercury (Planet)

Mercury, the closest object to the Sun, is a small, bleak planet. Because of the Sun's intense glare, it is difficult to observe Mercury from Earth.

## Metabolic Disorders

How are your enzymes working today? Enzymes are chemical compounds that increase the rate at which reactions take place in a living organism.

## Metabolism

Metabolism refers to all of the chemical reactions that take place within an organism by which complex molecules are broken down to produce energy and by which energy is used to build up complex molecules. An example of a metabolic reaction is the one that takes place when a person eats a spoonful of sugar.

## Metamorphosis

Metamorphosis is a series of changes through which an organism goes in developing from an early immature stage to an adult. Most people are familiar with the process, for example, by which a butterfly or moth emerges from a chrysalis (cocoon) in its adult form or a frog or toad passes through its tadpole stage.

## Meteor and Meteorite

Meteors, also known as "shooting stars," are fragments of extraterrestrial material or, more often, small particles of dust left behind by a comet's tail. We encounter meteors every time Earth crosses the path of a comet or the debris left behind a comet.

## Metric System

The metric system of measurement is an internationally agreed-upon set of units for expressing the amounts of various quantities such as length, mass, time, and temperature. As of 1994, every nation in the world has adopted the metric system, with only four exceptions: the United States, Brunei, Burma, and Yemen (which use the English units of measurement).

## Microwave Communication

A microwave is an electromagnetic wave with a very short wavelength, between .039 inches (1 millimeter) and 1 foot (30 centimeters). Within the electromagnetic spectrum, microwaves can be found between radio waves and shorter infrared waves.

## Migration

In biology, the term migration refers to the regular, periodic movement of animals between two different places. Migration usually occurs in response to seasonal changes and is motivated by breeding and/or feeding drives.

## Minerals

Minerals are the natural, inorganic (nonliving) materials that compose rocks. Examples are gems and metals.

## Mining

Mining is the process by which commercially valuable mineral resources are extracted (removed) from Earth's surface. These resources include ores (minerals usually containing metal elements), precious stones (such as diamonds), building stones (such as granite), and solid fuels (such as coal).

## Mole

In chemistry, a mole is a certain number of particles, usually of atoms or molecules. In theory, one could use any number of different terms for counting particles in chemistry.

## Molecular Biology

Molecular biology is the study of life at the level of atoms and molecules. Suppose, for example, that one wishes to understand as much as possible about an earthworm.

## Molecule

A molecule is a particle consisting of two or more atoms joined to each other by means of a covalent bond. (Electrons are shared in covalent bonds.) There are a number of different ways of representing molecules.

## Mollusks

Mollusks belong to the phylum Mollusca and make up the second largest group of invertebrates (animals lacking backbones) after the arthropods. Over 100,000 species of mollusks have been identified.

## Momentum

The momentum of an object is defined as the mass of the object multiplied by the velocity of the object. Mathematically, that definition can be expressed as p = m · v, where p represents momentum, m represents mass, and v represents velocity.

## Monsoon

A monsoon is a seasonal change in the direction of the prevailing wind. This wind shift typically brings about a marked change in local weather.

## Moon

The Moon is a roughly spherical, rocky body orbiting Earth at an average distance of 240,00 miles (385,000 kilometers). It measures about 2,160 miles (3,475 kilometers) across, a little over one-quarter of Earth's diameter.

## Mounds, Earthen

Earthen mounds are raised banks or hills built by prehistoric humans almost entirely out of soil or earth. Found in many different parts of the world, these mounds vary in size and shape, and most were built by ancient peoples as burial places or to serve some ceremonial purpose.

## Mountain

A mountain is any landmass on Earth's surface that rises to a great height in comparison to its surrounding landscape. Mountains usually have more-or-less steep sides meeting in a summit that is much narrower in width than the mountain's base.

## Multiple Personality Disorder

Multiple personality disorder (MPD) is a chronic (recurring frequently) emotional illness. A person with MPD plays host to two or more personalities (called alters).

## Multiplication

Multiplication is often described as repeated addition. For example, the product 3 × 4 is equal to the sum of three 4s: 4 + 4 + 4.

## Muscular System

The muscular system is the body's network of tissues that controls movement both of the body and within it (such as the heart's pumping action and the movement of food through the gut). Movement is generated through the contraction and relaxation of specific muscles.