From http://funprogramming.org/30-Multiple-rotating-objects-and-reset-matrix.html

multiple grid

size(400, 400);
noStroke();
background(23, 100, 240);
 
float x = 0;
while (x < width) {
  float y = 0;
  while (y < height) {
    // in rare cases, paint using red color
    if (random(100) > 98) {
      fill(255, 0, 0);
    } 
    else {
      // but usually pick a random gray color
      fill(random(0, 105));
    }
    ellipse(x + 20, y + 20, 44, 44);
    y = y + 10;
  }
  x = x + 50;
}