Sand

A cellular automaton on a small grid. Sand falls and slumps into piles, water finds its level and sand sinks through it. Wall is the one material with no physics — it hangs wherever you paint it, so you can build ledges and funnels for everything else to fall through. Nothing paints over anything else: to move a pile, erase it first. Drop seeds onto wet ground and they drink the water, creeping upward into a patch of green.