  The internal (or residual) stress is among the key notions to describe the
state of the systems far from equilibrium. Such stress is invisible on the
macroscopic scale where the system is regarded as a blackbox. Yet
nonequilibrium macroscopic operations allow to create and observe the internal
stress. We present in this lecture some examples of the internal stress and its
operations. We describe the memory effect in some detail, the process in which
the history of past operations is recalled through the relaxation of internal
stress.

Soft Condensed Matter;Statistical Mechanics