2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/140250We show a very simple yet rigorous derivation of the invariance of the space-time interval (and hence the whole special relativity) just from the isotropy, homogeneity and a principle of relativity, without the need of the speed of light axiom. This article is intended as a textbook explanation of the special relativity.14 pages, submitted to Eur. J. PhysClassical Physicstext