void setup() {
  size(500,500);
  smooth();
 
}

void draw() {
  // draw magenta background //
   background(255,0,255);
   
   // set rect's to center mode //
   rectMode(CENTER);
   
   // draw 2 rect's on screen //
   rect(mouseX,mouseY,50,50);
   fill(255);
   rect(mouseX,mouseY,25,25);
   fill(0);
}