During processing some molecules may not recover to their lowest energy level. That means that they have been retained in a higher energy state than they would like to be in and as soon as they have a chance they will recover to their lower energy level. Through a solid-stae process driven by thermal energy, they slowly move back to their equilibrium positions. The result is a change in the shape or contour of the solid as a gross manifestation of the rearrangement in atomic or molecular position. The material warps or distorts. Such a relief of stress is known as relaxation.