A cover mounted on a top surface of either mobile phone or PDA includes a top member; a bottom member fixedly mounted on the top surface and including two inner slide members on both sides; an intermediate member fixedly secured to the top member and partially disposed on the bottom member, the intermediate member including two outer slide members on both sides; and an elastic assembly including left and right elastic members including a triangular section at one end secured to the bottom member, a U-shaped intermediate section, and a straight section at the other end secured to the intermediate member. Pulling the top member will slide the intermediate member away from the bottom member by moving the outer slide members on the inner slide members by expanding the elastic assembly. Releasing the top member will elastically return the intermediate member to its original position.