One speaks of water existing in three states or phases: solid, liquid, and gas, known to us simply as ice, water, and steam. Temperature and pressure determine the phase of water molecules. Similarly, protons and neutrons exhibit different phases depending on the local nuclear temperature and density. Normal nuclei appear to be in the liquid phase. Different types of nuclear matter include neutron stars, the early universe, a nucleon gas, or a quark-gluon plasma. Scientists study these phases by colliding beams of accelerated particles to produce extreme conditions. At this time, the quark-gluon plasma has not been identified in any experiment.