A spin glass is a disordered magnet where exchange couplings J_ij are random (typically Gaussian). Competing ferromagnetic and antiferromagnetic bonds create frustration — no configuration can satisfy all bonds simultaneously. The free energy landscape has exponentially many local minima (metastable "states"). Parisi's replica symmetry breaking (RSB) solution (1979, Nobel 2021) computes the partition function by introducing n→0 replicas: the order parameter q(x) is a function, not a number, encoding the hierarchical ultrametric structure of states. The overlap distribution P(q) is broad in the glass phase (unlike a ferromagnet where it has delta-function peaks). Right panel: overlap matrix Q_{ab} = ⟨s_i^a s_i^b⟩ between replica pairs — block structure signals RSB levels. Left: spin configurations across replicas; bonds are colored by J_ij sign (red=antiferro, blue=ferro).