Help creating simple animation

[TfGuy44]

void setup() {
  size(640, 360);
}

void draw() {
  background(0); // create black background
  fill(102); //set color to gray
  if ( millis()%6000 < 3000 ) {
    rect(81, 81, 63, 63); // draw a rectangle
  } else {
    rect(81, 81, 150, 150); // draw slightly larger rectangle
  }
}