Gas particles expand into a vacuum — watch entropy (Boltzmann H-theorem) increase monotonically. Reverse: entropy never spontaneously decreases. Visualize phase-space coarse-graining.
Boltzmann's H-theorem: H = ∫f ln(f) dv decreases monotonically under elastic collisions (Boltzmann equation). Despite time-reversible microscopic laws (Loschmidt paradox), macroscopic irreversibility emerges from coarse-graining — the foundation of statistical mechanics.