float dx,dy,delta;
float x1;
float y1;
void setup(){
  size (200,200);
  background(255);
}

void draw(){
  for (int y=0; y<height; y=y+10) {
    for (int x=0; x<width; x=x+10){
      smooth();
      pushMatrix();
      rect(x,y,8,8);
       fill (0);
      popMatrix();
      rect(dx,dy,8,8);
      dx=mouseX-x1;
      dy=mouseY-y1;
      delta=sqrt(dx*dx+dy*dy);
     fill(255);
    }
  }
}