2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/87868We consider an isolated gaseous system, divided in two parts by an adiabatic movable frictionless internal wall undergoing Brownian motion. We show how this kind of motion can lead to a substantial decrease of the system entropy. This surprising result does not violate Boltzmann's H-theorem since the molecular chaos assumption underlying H-theorem is not fulfilled by our system.Classical Physicstext