Smoluchowski's coagulation equation describes cluster aggregation: dc_k/dt = ½∑K(i,j)c_i c_j − c_k∑K(k,j)c_j. With multiplicative kernel K(i,j)=ij, a gelation transition occurs at finite time t_gel=1: the weight-average cluster size diverges as (t_gel−t)⁻¹. Post-gel, a "giant cluster" (gel) coexists with finite clusters following power-law distributions.