A cube is one of the five regular solids. All its faces are regular polygons (squares), and all the vertices are alike. One of the basic properties of the cube is that cubes pack 3-dimensional space. This makes them good for packaging, or any other application where it is helpful for congruent solids to fit together. This cubic packing structure occurs naturally in the crystals of salt, NaCl. If we think of the atoms being placed at the points of an integer lattice (x, y, z), then we might place the sodium (Na) atoms at points with x + y + z even, and the chlorine (Cl) atoms at the remaining points. The cubic nature of salt crystals is quite striking.
