Gravitational lensing is the bending of light by gravity, predicted by general relativity. A massive foreground object, such as a galaxy or cluster, distorts and magnifies the light of more distant objects behind it, sometimes smearing them into arcs or complete rings of light called Einstein rings. Lensing lets astronomers weigh dark matter, study extremely distant galaxies, and test gravity itself, and AI now finds new lenses in survey images faster than humans can.