2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/106352We give a short proof of Bing's characterization of $S^3$: a compact, connected 3-manifold $M$ is $S^3$ if and only if every knot in $M$ is isotopic into a ball.To appear in Proceedings of the AMS, 2 pagesGeometric Topology57M40; 57N12text