If two marbles collide, they rebound with less total kinetic energy than they had before the collision. Each is warmed slightly by the collision, since the lost energy has spread as internal energy within the marbles. Some of the original macroscopic ordered energy has been converted to microscopic disordered energy. This is a simple example of the equipartition theorem at work. Energy concentrated initially in one mode of motion tends to distribute itself evenly over all of the available modes of motion.