function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  x = width/2;

  y = height/2;

  f = 255;

  s = 50;

  fill(255);

  rect(0, 0, 100,100);//top left

  rect(300, 0, 100,100);//top right

  rect(0, 300, 100,100);//bottom left

  rect(300, 300, 100,100);//bottom right

  if( mouseX > 0 && mouseX < 100 && mouseY > 0 && mouseY < 100){

      f = 0; // change fill to light blue

  } else if (mouseX > 300 && mouseX < width && mouseY > 0 && mouseY < 100){

      s = 100; // increase size     

  }

    fill(f, 255, 255)

  ellipse(x, y ,s);//center

}