An interaction in which participation inevitably leads to overall loss, even though one agent may gain. So a person losing must lose more than the person winning. Because of the principle of conservation of overall energy (namely “the total energy of an isolated system must remain constant”) and as some kinetic energy in a system is necessarily converted into thermal energy and lost to the system when the system interacts, the kinetic energy in an isolated system will dissipate over time. The system will be “negative sum” — it will need more energy injected to keep operating. Hence, perpetual motion machines are not possible.