var y = 20;

var changeX = 1;

var changeY = 0;

​

function setup() {

  createCanvas(400, 400);

  background(0);

  stroke (225);

}

​

function draw() {

  background(0,0,0,5);

  ellipse(x, y, 30,30);

  x = x + changeX;

  y = y + changeY;

  changeY = changeY + .02;

  if(x > 400){

    changeX = -1;

  }

  if(x < 0){

    changeX = 1;

  }

  if(y > 400){

    changeY = -changeY;

  }  

}