Freezing of two-dimensional hard disks.

We argue that structural rearrangements experienced by an assembly of hard disks under increasing disk density are accompanied by the mutual caging of each disk by its three alternating Voronoi nearest neighbors. This caging becomes effective at a packing fraction eta=pisqrt[3]8 approximately 0.680 when the average gap width between neighboring disks in the system shrinks to about 15% of the disk diameter. The freezing occurs when the fraction of caged disks is about 40%.