<Table> Matter <Tr> <Td_colspan="2"> </Td> </Tr> <Tr> <Td_colspan="2"> Matter is usually classified into three classical states, with plasma sometimes added as a fourth state . From top to bottom: quartz (solid), water (liquid), nitrogen dioxide (gas), and a plasma globe (plasma). </Td> </Tr> </Table> <Tr> <Td_colspan="2"> Matter is usually classified into three classical states, with plasma sometimes added as a fourth state . From top to bottom: quartz (solid), water (liquid), nitrogen dioxide (gas), and a plasma globe (plasma). </Td> </Tr> <P> In the classical physics observed in everyday life, matter is any substance that has mass and takes up space by having volume . This includes atoms and anything made up of these, but not other energy phenomena or waves such as light or sound . More generally, however, in (modern) physics, matter is not a fundamental concept because a universal definition of it is elusive; for example, the elementary constituents of atoms may be point particles, each having no volume individually . </P> <P> All the everyday objects that we can bump into, touch or squeeze are ultimately composed of atoms . This ordinary atomic matter is in turn made up of interacting subatomic particles--usually a nucleus of protons and neutrons, and a cloud of orbiting electrons . Typically, science considers these composite particles matter because they have both rest mass and volume . By contrast, massless particles, such as photons, are not considered matter, because they have neither rest mass nor volume . However, not all particles with rest mass have a classical volume, since fundamental particles such as quarks and leptons (sometimes equated with matter) are considered "point particles" with no effective size or volume . Nevertheless, quarks and leptons together make up "ordinary matter", and their interactions contribute to the effective volume of the composite particles that make up ordinary matter . </P>

Matter can be described as the amount of