Love this. I also was wondering how to do this cleanly, and in my mind it seemed to make sense to apply the rules to the sand, but onto a blank canvas so as not

Love this. I also was wondering how to do this cleanly, and in my mind it seemed to make sense to apply the rules to the sand, but onto a blank canvas so as not to have the sand interfere with the sand below. Doing it from the bottom up solves this problem, however I think in a general sense (eg. with different sources of gravity it would become more tricky!