text: class Stone{
      constructor(x, y, r){
          var options = {
              isStatic: false,
              restitution: 0.8
          }
          this.radius = r;
          this.body = Bodies.circle(x, y, r, options);
          World.add(world, this.body);
      }
      display(){
          var pos = this.body.position;
          ellipseMode(RADIUS);
          fill("white");
          ellipse(pos.x, pos.y, this.r, this.r);
      }
  }