Any interaction between objects that lasts for a short period of time is termed as collision. During a collision, the objects constituting the system exert forces on each other. In the absence of any external force, or if the effect of net external force is zero, the system's momentum is said to be conserved. The collision in a system whose momentum is conserved can be categorized as either elastic or inelastic, based on the nature of the interaction. A collision in which there is no net change in the system's kinetic energy is termed as an elastic collision. For example, consider a system composed of gas molecules. In the absence of any external force, the molecules continue to collide with each other and with the walls of the container. The velocity of individual molecules may change, but the total kinetic energy of the system would remain the same before and after the interaction.