Elemental Sandbox
Paint with sand, water, fire and lava, and watch them react.
The brief
Build a falling-sand sandbox where I paint with at least 12 interacting elements: sand, water, oil, fire, smoke, steam, lava, stone, ice, plant, acid and wood. Interactions should be believable and surprising: lava and water make stone and steam, fire races through oil and wood, plants grow toward water, ice melts near heat, and acid eats most things. Open on a scene that is already mid-reaction, and provide an element palette, brush size, pause and clear. Keep it at 60 fps on a simulation grid of at least 400×300 cells.