Depending on pressure and temperature, a substance can exist in three phases (gas, liquid or solid). Water for example exists as steam, water or ice. The transition between phases may be noticed as abrupt changes in pressure and temperature. At normal pressures both 4He and 3He refuse to freeze no matter how low the temperature. The explanation is that no atoms can be completely at rest (zero point motion) and that the attractive force between helium atoms is too weak to make them come to rest. However, if the pressure is increased to about 3 MPa (30 atmospheres) both fluids become solids at very low temperatures. 4He is the more common isotope of helium. The figure shows the phase diagram of 4He at low temperatures. 4He remains liquid at zero temperature if the pressure is below 2.5 MPa (approximately 25 atmospheres). The liquid has a phase transition to a superfluid phase, also known as He-II, at the temperature of 2.17 K (at vapor pressure). The solid phase has either hexagonal close packed (hcp) or body centered cubic (bcc) symmetry The phase diagram of 3He is shown in the figure. Note the logarithmic temperature scale. The dot in the lower right hand corner denotes room temperature and pressure. There are two superfluid phases of 3He, A and B. The line within the solid phase indicates a transition between spin-ordered and spin disordered structures (at low and high temperatures, respectively).