function keyListener(e){

        if(e.keyCode==37 && paddleLeft > 0){
            //keyCode 37 is left arrow
            paddleLeft -= 10;
            paddle.style.left = paddleLeft + 'px';
        }
        if(e.keyCode==39 && paddleLeft < 436){
            //keyCode 39 is right arrow
            paddleLeft += 10;
            paddle.style.left = paddleLeft + 'px';
        }
        //FYI - keyCode 38 is up arrow, keyCode 40 is down arrow
    }

    function start(){
        //game loop
        detectCollisions();
        render();
        difficulty();

        //end conditions
        if(ballTop < 470){
            //still in play - keep the loop going
            timer = setTimeout('start()',50);
        }
        else{
            gameOver();
        }
    }

    function detectCollisions(){
        //just reflect the ball on a collision
        //a more robust engine could change trajectory of ball based
        //on where the ball hits the paddle
        if(collisionX())
            dx = dx * -1;
        if(collisionY())
            dy = dy * -1;
    }

    function collisionX(){
        //check left and right boundaries
        if(ballLeft < 4 || ballLeft > 285)
            return true;
        return false;
    }

    function collisionY(){
        //check if at top of playing area
        if(ballTop < 4)
            return true;
        //check to see if ball collided with paddle
        if(ballTop > 400){
            if(ballLeft > paddleLeft && ballLeft < paddleLeft + 64)
                return true;
        }
        return false;
    }