Before the 20th centuaary, the termmatterincludedordinary mattercomposed of atoms and excluded other energy phenomena such aslightorsound. This concept of matter may be generalized from atoms to include any objects havingmass even when at rest, but this is ill-defined because an object'smasscan arise from its (possibly massless) constituents' motion and interaction energies. Thus, matter does not have a universaldefinition, nor is it a fundamental concept inphysicstoday. Matter is also used loosely as a general term for the substance that makes up all observablephysical objects.[1][2]All the objects from everyday life that we can bump into, touch or squeeze are composed ofatoms. This atomic matter is in turn made up of interactingsubatomic particlesusually anucleusofprotonsandneutrons, and a cloud of orbitingelectrons.[3][4]Typically, science considers these composite particles matter because they have both rest mass and volume. By contrast,massless particles, such asphotons, 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 asquarksandleptons(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.Matter commonly exists in fourstates(orphases):solid,liquidandgas, andplasma. However, advances in experimental techniques have revealed other previously theoretical phases, such asBoseEinstein condensatesandfermionic condensates. A focus on an elementary-particle view of matter also leads to new phases of matter, such as thequarkgluon plasma.[5]For much of the history of thenatural sciencespeople have contemplated the exact nature of matter. The idea that matter was built of discrete building blocks, the so-calledparticulate theory of matter, was first put forward by the Greek philosophersLeucippus(~490 BC) andDemocritus(~470380 BC).[6]Matter should not be confused with mass, as the two are not quite the same in modern physics.[7]For example, mass is aconserved quantity, which means that its value is unchanging through time, within closed systems. However, matter isnotconserved in such systems, although this is not obvious in ordinary conditions on Earth, where matter is approximately conserved. Still,special relativityshows that matter may disappear by conversion into energy, even inside closed systems, and it can also be created from energy, within such systems. However, becausemass(like energy) can neither be created nor destroyed, the quantity of mass and the quantity of energy remain the same during a transformation of matter (which represents a certain amount of energy) into non-material (i.e., non-matter) energy. This is also true in the reverse transformation of energy into matter.Different fields of science use the term matter in different, and sometimes incompatible, ways. Some of these ways are based on loose historical meanings, from a time when there was no reason to distinguish mass and matter. As such, there is no single universally agreed scientific meaning of the word "matter". Scientifically, the term "mass" is well-defined, but "matter" is not. Sometimes in the field of physics "matter" is simply equated with particles that exhibit rest mass (i.e., that cannot travel at the speed of light), such as quarks and leptons. However, in bothphysicsandchemistry, matter exhibits bothwave-like andparticle-like properties, the so-calledwaveparticle duality