Falling Sand simulation

I'm trying to re-create a 'falling sand' simulation, similar to those various web toys that are out there doing the same thing - and I'm failing pretty hard. I'm not really sure where to begin. I'm trying to use cellular automata to model the behavior of the sand particles, but I'm having trouble figuring out how to make the direction in which I update the 'world' not matter...

For example, one of the particle types I'd like to have is Plant. When Plant comes in contact with Water, Plant turns that Water particle into another Plant particle. The problem here though is that if I'm upd开发者_开发问答ating the game world from top to bottom and left to right, then a Plant particle placed in the middle of a sea of Water particles will immediately cause all of the Water particles to the right and below that new Plant particle to turn into Plants. This is not the behavior I am expecting. =(

One straightforward solution is to not do each iteration in-place. Instead, every time you update the world, create a copy of it... then look at the original, but update the copy. That way the order of updating does not matter any more, because you are completely disregarding your updates while you're looking for particles.

Don't program it in a sequential way (looping over all particles) but use real simulation programming techniques in which every particle is treated as an individual object/agent that obeys the laws of physics and that can act (run) asynchronously and respond to "events" (interactions with other particles).

If making every sand particle a separate object is too fine-grained, then divide the world into small blocks of let's say 1000 particles and simlute the behavior of these blocks instead.