I found this text As the ball's surface dents during an impact, these polymer molecules move about and begin to exert forces on one another (storing energy in the process). As the ball rebounds, these molecules release their stored energy and push the ball back into the air. I'm not sure where this energy is stored. Is it just transformed from kinetic to potential? is it stored in the orbits of the electrons? do the electrons descend from a higher to a lower orbit when they hit the wall? if so, i don't see why.