The cellular network produced by annealing of a two-dimensional polycrystal. The grain boundaries consist of curves meeting at triple points. Occasionally two triple points merge, eliminating the curve between them (this has just happened at location A, where four curves meet at a point). If three or more triple points merge simultaneously, an entire grain can disappear. This event might happen soon at location B, where a grain is very small. Such singular events change the network irreversibly.