The jamming transition is a phase transition from a flowing granular state to a rigid, amorphous solid as packing fraction φ exceeds a critical value φ_c ≈ 0.64 (random close packing in 2D: φ_c ≈ 0.84). Below φ_c: grains flow around each other; above: force chains percolate across the system, transmitting stress. The transition is continuous but with diverging length scales — analogous to a critical point, but at T=0. The phase diagram has three axes: temperature T, applied stress Σ, and 1/φ — jamming occurs at the origin (T=0, Σ=0, φ=φ_c). Observe the probe ball: in the unjammed phase it sinks; in the jammed phase it is supported by the force network (shown in yellow).