The reason why an athlete puts sand or cushion on the ground while high jumping is to reduce the impact of their landing. When an athlete jumps, they generate a great deal of kinetic energy, which must be dissipated upon landing. Without proper landing techniques, this energy can cause significant stress on the athlete's body, leading to injury. By placing sand or cushion on the ground, the athlete can reduce the impact of their landing and dissipate the kinetic energy more gradually. The sand or cushion will compress upon impact, absorbing some of the energy and reducing the force of the landing. Furthermore, the sand or cushion can also provide a more stable landing surface, reducing the risk of slipping and further minimizing the impact of the landing.