float ang;

void setup() {
  size (200,200);
  background (255);
  framerate (50);
  rectMode(CENTER);
}
void draw () {
  background (255); 
  stroke(0,0,0,50);                 // black lines
  translate (100,100);              //move origin of red square

  line(0,-width, 0, width);         // draw vertical line
  line(-height, 0, height, 0);      // draw horizontal line

  noFill ();
  stroke(255,0,0,50);               // red square(static square)  
  rect (0,0,50,50);
  rotate (ang); 

  translate (-75,-75);              //move origin of black square
  stroke(0,0,0);                    // black square
  noFill ();
  rect (75,75,50,50);               // draw black square(moving square)

  ang += TWO_PI/360;
}