Matter is any physical substance that occupies space, has mass, is composed of atoms or, in the case of subatomic particles, is part of an atom and is convertible to energy. On Earth, matter appears in three clearly defined forms: solid, liquid, and gas whose varying structural characteristics are a function of the speeds at which its molecules move in relation to one another. A single substance may exist in any of the three phases: liquid water, for instance, can be heated to become steam, a vapor; or, when sufficient heat is removed from it, it becomes ice, a solid. These are merely physical changes, which do not affect the basic composition of the substance itself: it is still water. Matter, however, can and does undergo chemical changes, which (as with the various states or phases of matter) are an outcome of activity at the atomic and molecular level.
One of the characteristics of matter as noted in its definition above is that it is convertible to energy. We rarely witness this conversion; though as Albert Einstein (1879-1955) showed with his Theory of Relativity, it occurs in a massive way at speeds approaching that of light. Einstein's famous formula, E = mc2, means that every item possesses a quantity of energy equal to its mass multiplied by the squared speed of light. Given the fact that light travels at 186,000 mi (299,339 km) per second the quantities of energy available from even a tiny object traveling at that speed are enormous indeed. This is the basis for both nuclear power and nuclear weaponry, each of which uses some of the smallest particles in the known universe to produce results that are both amazing and terrifying.
Even in everyday life, it is still possible to observe the conversion of mass to energy, though only on a very small scale. When fire burns, that is, when wood experiences combustion in the presence of oxygen, and undergoes chemical changes, a tiny fraction of its mass is converted to energy. Likewise, when a stick of dynamite explodes, it too experiences chemical changes and the release of energy. The actual amount of energy released is, again, very small: for a stick of dynamite weighing 2.2 lb (1 kg), the portion of its mass that "disappears" is be equal to 6 parts out of 100 billion.
Actually, none of the matter in the fire or the dynamite blast disappears: it simply changes forms. Most of it becomes other types of matter, perhaps new compounds, and certainly new mixtures of compounds. A very small part, as we have seen, becomes energy. One of the most fundamental principles of the universe is the conservation of energy, which holds that within a system isolated from all other outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place. In this situation, some of the energy remains latent, or "in reserve" as matter, while other components of the energy are released; yet the total amount of energy remains the same.