​

function setup() {

  createCanvas(600, 400);

  frameRate(1);

}

​

function draw() {

  background(220);

​

  //if mouse inside third rectangle, hover becomes true

  if (mouseX > 400) {

    hover = !hover;

  }

​

  // if hover is true, fill 3rd box,

  // if hover is false, nofill 3rd box

  if (hover) {

    fill("red");

    rect((width / 3) * 2, 0, width / 3, height);

  } else {

    noFill();

    rect((width / 3) * 2, 0, width / 3, height);

  }

  //default no fill 2 box

  noFill();

  rect((width / 3) * 0, 0, width / 3, height);

  rect((width / 3) * 1, 0, width / 3, height);

}

​