int[][] particles;
int s = 200;

void setup() {
  smooth();
  size(2*s, 2*s);
  fill(#333333);
  noStroke();
  particles = new int[s][2];
  reset();
  frameRate(30);
}

void reset() {
  for (int i = 0; i < s; i++) {
    particles[i][0] = s/2;
    particles[i][1] = s/2;
  }
}

void mouseClicked() {
  reset();
}

void draw() {
  background(#cccccc);
  for (int i = 0; i < s; i++) {
    int x = particles[i][0];
    int y = particles[i][1];
    rect(2*x, 2*y, 2, 2);
    float r = random(0, 2);
    if (r < 1) x--;
    else x++;
    r = random(0, 2);
    if (r < 1) y--;
    else y++;
    particles[i][0] = x;
    particles[i][1] = y;
  }
}