English: Schematic of a small part of a growing crystal. The crystal is of (blue) cubic particles on a simple cubic lattice. The top layer is incomplete, only ten of the sixteen lattice positions are occupied by particles. A particle in the fluid (shown with red edges) is joining the crystal, growing the crystal by one particle. It is joining the lattice at the point where its energy will be a minimum, which is in the corner of the incomplete top layer (on top of the particle shown with yellow edges). Its energy will be a minimum because in that position it is three neighbours (one below, one to its left and one above right) which it will interact with. All other positions on an incomplete crystal layer have only one or two neighbours.