submission by jtoy
function setup(){
  var myCanvas = createCanvas(windowWidth,windowHeight); //you must keep this line unmodified
  ballx = width/2
  bally = height/2
  speed = 1
  background_color = color(random(255),random(255),random(255))
}

function draw(){
  background(background_color)
  ellipse(ballx,bally,40,40)
  if(keyIsDown(LEFT_ARROW)){
     ballx -= speed
  }
  if(keyIsDown(RIGHT_ARROW)){
     ballx += speed
  }

  if(keyIsDown(UP_ARROW)){
     bally -= speed
  }
  if(keyIsDown(DOWN_ARROW)){
     bally += speed
  }

  for(i=5;i>0;i--){
     fill(i*20,255,100)
     rect(mouseX,mouseY,   i*20,i*20)
   }
}

function mousePressed(){
   background_color = color(random(255),random(255),random(255))
   if(mouseX > width/2){
      speed+=1
    }else{
       speed -= 1
   }

}