Granular gases lose kinetic energy through inelastic collisions (restitution coefficient e < 1), leading to Haff's cooling law: T(t) ∝ (1 + t/t₀)⁻². Clustering instability emerges at late times.
Haff's law (1983): granular temperature decays algebraically. Below e≈0.8, clustering instability creates dense cold regions.