Consider a stack of books placed on a flat desk surface. The books sit squarely one atop the other, edges aligned, forming a straight tower. Right after stacking, the setup appears solid, with even height from base to top.
The stack holds this vertical form steadily in the moments following placement. Its shadow falls directly behind it under room light.
As hours pass, the top book shifts minutely to one side. The movement spreads downward, turning the entire stack into a slight lean.
Time continues, and the tilt grows. Now the upper books overhang the base, altering the stack's profile from straight to angled.
Initially compact and upright, the stack later spreads horizontally. Light hits the leaned side differently, casting a slanted shadow. The top surface drifts outward from its starting point.
A book stack reveals its changing nature over time. What begins as a fixed pillar eases into a tilted form as conditions progress.