2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/34955We argue that every finite piecewise smooth spacelike two-surface of spacetime possesses an entropy which is, in natural units, one quarter of its area. However, the thermodynamical properties of a two-surface become apparent only to the observers having that two-surface as a horizon. Consequences of this result are discussed.3 pages, no figuresGeneral Relativity and Quantum Cosmologytext