As this is my first actual attempt at programming, please forgive my idiocy.  I am trying to make a 15x15 grid where I can color each individual square black at a mouseclick.  I can fairly easily make a 15x15 grid, but when I click it turns the whole grid black rather than any given square.  Here is that code:

void setup() {
  size(1020, 1120);
  background(255);
}

void draw() {
  strokeWeight(5);
  rect(400, 20, 600, 600);
  strokeWeight(1);
  for (int a=400; a<=960; a=a+40) {
    for (int b=20; b<=580; b=b+40) {
       rect(a, b, 40, 40);
    }
  }
}


void mousePressed() {
  if (mouseButton == LEFT) {
    fill(0);
  } else if (mouseButton == RIGHT) {
    fill(255);
  } else {
    fill(126);
  }
}

I have since figured out I need to make an object for each box so that i can vary the fill individually (am I right?).  I can make an object, but I dont want to individually define 225 separate objects typing in the locations manually, and I was wondering if there was a way to create a slew of objects, similar to the way I create a bunch of rectangles up above?