Edit in JSFiddle

$(function() {
        
        var canvas = document.getElementById("canvas");
        var liveBalls = new Array();
        var escapedCount = 0;
    
        function createBall() {
            // create the ball image
            image = new Image();
            image.src = 'http://dl.dropbox.com/u/19593893/CDN/ball.png';
            
            // push a new ball object into the liveBalls array
            liveBalls.push({ image: image, 
                             x: Math.floor(Math.random()*580), 
                             y: 640, 
                             index: 0, 
                             speed: Math.floor(Math.random()*5) });
        }

        function draw() {

            // get the context we are going to draw to
            var context = canvas.getContext('2d');

            // clear the context to make sure we have a clean canvas
            context.clearRect(0, 0, 640, 480);
            
            // set the beatiful beach background
            var background = new Image();
            background.src = 'http://dl.dropbox.com/u/19593893/CDN/beach.jpg';
            
            // draw the background to the canvas
            context.drawImage(background, 0, 0, 640, 480);

            // keep track of our escaped balls with this array
            var escapedBalls = new Array();
            
            // for each of our live balls, see if the ball is still on the canvas, if it is,
            // move it up by the random speed of the ball.
            $.each(liveBalls, function(index, value) {
                this.index = index;
                this.y -= this.speed;
                if (this.y > -64) {
                    context.drawImage(this.image, this.x, this.y, 45, 45);
                }
                else {
                    // the ball is off the screen so add it to the escaped balls
                    // collection and increment the score
                    escapedBalls.push(this);
                    escapedCount++;
                    $("#score span").text(escapedCount);
                }
            });
            
            // remove each of the escaped balls from the liveBalls array
            $.each(escapedBalls, function(index, value) {
                liveBalls.splice(this.index, 1);
            })
            
            // delete the array - we don't need it anymore
            delete escapedBalls;

            // this is the call to the function which will call draw again
            // at whatever the current framerate is.
            get_animation_frame()(draw, canvas);
        }

        // get the framerate at 60 FPS or whatever the current framerate 
        function get_animation_frame() {
              return window.requestAnimationFrame || 
                     window.webkitRequestAnimationFrame || 
                     window.mozRequestAnimationFrame || 
                     window.oRequestAnimationFrame || 
                     window.msRequestAnimationFrame || 
              function(callback, element) {
                return window.setTimeout(callback, 1000 / 60);
          };
        };
    
        function detectCollision(x,y) {
            
            // enumerate through the balls and see if any of them collide with our mouse click
            var collisionDetected = false;
            var collisionIndex = 0;
            
            // collision detection is rough here, could be much better.  Basically detecting
            // if the mouse click falls inside a sqaure that's roughly the same size as the ball.
            $.each(liveBalls, function(index, value) {
                if (x > this.x && x < this.x + 64 && y > this.y + 20 && y < this.y + 100) {
                    collisionDetected = true;
                    collisionIndex = this.index;
                }
            });
            
            // if we detected a collision, remove the object.
            if (collisionDetected) {
                liveBalls.splice(collisionIndex, 1);
            }
        }
    
        // bind the mouse click event on the page to detect a collision
        $(document).click(function(e){
            detectCollision(e.pageX,e.pageY);
        });
    
        // initialize the draw loop
        draw();
    
        // set an interval for creating random balls
        setInterval(createBall, 500);

    });

<canvas id="canvas" width="640" height="480">
    No Canvas Support!
</canvas>
<div id="score">Escaped Balls: <span><strong></strong></span></div>