void setup() {
  size(400, 400);
  background(0);
  noStroke();

  // Outer loop ? columns
  for (int x = 0; x < width; x += 40) {

    // Inner loop ? rows
    for (int y = 0; y < height; y += 40) {

      // Random size for each circle
      float circleSize = random(10, 30);

      // Random color for each circle
      fill(random(255), random(255), random(255));

      // Draw circle centered in each grid cell
      ellipse(x + 20, y + 20, circleSize, circleSize);
    }
  }
}