var y = 200;

var d = 50;

var state = false;

​

function setup() {

  createCanvas(600, 400);

}

​

function draw() {

  if (state) {

    background(0);

  } else {

    background(255);

  }

  ellipse(x, y, d, d);

}

​

function mousePressed() {

  if (dist(mouseX, mouseY, x, y) < d/2) {

    state = !state;

  }

}

​