how do you know the two colors of sand will weigh the same amount ? if not, over time the heavy sand will sink and "sort" the color. Over time the sand could (

how do you know the two colors of sand will weigh the same amount ? if not, over time the heavy sand will sink and "sort" the color. Over time the sand could (perhaps) grind itself into flour and not exude the same mechanical properties anymore. Finally, even if that worked, it might be good for what, 2,3, maybe 5 samples per second. If you take too many pictures before the sand moves, it's no longer random. What if I need millions of random numbers ?