float x;
float vx;

void setup(){

  background (80);

  x=50;
  vx= 0.2;

  smooth();
  stroke (0,255,240);
  fill(0,255,240);
  ellipse (50,50,5,5);
  strokeWeight (5); 
  line (20,15,20,85);
  line (80,15,80,85);
}

void draw(){

  x=x+vx;
  if ((x > 78) || (x < 22)) {
    vx=vx*-1;
  }
  ellipse (x,50,1,1);
}