let d;

function setup() {

  cnv = createCanvas(100, 100);

  cnv.mouseOver(changeGray);

  d = 10;

}

​

function draw() {

  ellipse(width / 2, height / 2, d, d);

}

​

function changeGray() {

  d = d + 10;

  if (d > 100) {

    d = 0;

  }

}