A crystal is a solid substance whose atoms, molecules, or ions are organised in a repeating, ordered pattern that spans all three spatial dimensions. A crucial step in the crystallisation process is called crystal growth, which involves incorporating additional atoms, ions, or polymer strings into the crystalline lattice's distinctive configuration. A homogeneous or heterogeneous (surface catalysed) nucleation stage usually precedes the growth, unless a "seed" crystal that was inserted specifically to kick-start the growth was already present. The process of crystallisation results in a crystalline material with tightly packed atoms or molecules that have fixed spatial locations with respect to one another. The structural rigidity and extremely high resistance to deformation (i.e., changes in shape and/or volume) that define the crystalline state of matter are distinctive features. Young's modulus and shear modulus of elasticity are two properties that the majority of crystalline solids have in considerable amounts. In contrast, the majority of liquids or fluids have a low shear modulus and frequently demonstrate the ability to flow macroscopically in a viscous manner.