Build a falling-sand physics toy in the browser (single HTML page, no external assets). Sand, water, lava, stone, wood, fire, smoke, steam and plants that all interact: lava turns water to steam and stone, fire spreads through wood, plants grow in wet sand. It starts with an hourglass pouring and a volcano erupting by itself, and you paint with the mouse from a material palette. It should feel mesmerizing the moment it loads.