An isolated system is a physical or thermodynamic system that is either so far removed from other systems that it does not interact with them or is enclosed by rigid immovable walls through which neither mass nor energy can pass. An isolated system obeys the conservation law that its total energy-mass stays constant. In thermodynamics, an isolated system does not allow either matter or energy to enter or leave/Thermochemistry/Basic_Definitions). There are two criteria for the presence of a net external force that alters the momentum of the system, and if these criteria are not met, the system can be considered an isolated system. The two criteria are that the system must be free from the influence of a net external force that alters the momentum of the system, and the only forces that contribute to the momentum change of an individual object are the forces acting between the objects themselves. An isolated system is a useful model approximating many real-world situations, but strictly and ideally isolated systems do not actually occur in experiments or in nature.