title: Hourglass for iPhone
body: <p>I'm trying to exploring iOS application right now. I got a simple idea, which is to create an hourglass application. I did found some apps on the App Store, but the animation is not quite satisfying. Lets say, if I want to create a smooth animation of the sand falling down, the gravity, is there any advice on where should I start? I am thinking to use OpenGL ES for the animation. not sure if this is a good idea, since I do not know how to keep the sand in the hourglass, and each grain falls perfectly fine. is there any algorithm that I should look into?</p>