void setup() {
  size(400, 400);
  smooth();
  background(0);
}

void draw() {
  if (mousePressed) {
    drawRing(random(13, 50));
  }
  else {
    drawRing(random(50, 100));
  }
}

void drawRing(float s) {
  float x = random(width);
  float y = random(height);

  /*
  // Uncomment to add shadow
   fill(0, 160);
   noStroke();
   ellipse(x + 5, y + 5, s, s);
   */

  stroke(random(100, 255), 0, random(150, 255));
  fill(0);
  ellipse(x, y, s, s);
  ellipse(x, y, s - 5, s - 5);
}