Falling Sand

A falling sand game is a box of square cells, each holding one material, with a short list of rules about what each material does to the cell below it. Sand falls and piles up. Water falls and spreads out. Oil floats on water and catches fire. Plants drink water, burn, and turn to smoke. Acid eats through almost anything and is used up doing it. There is no goal and no way to lose — the whole of it is pouring things in and seeing what happens.

How to play

Pick a material, pick a brush size, and drag on the box to paint it in. Holding the brush still over sand, water, oil, acid or fire keeps pouring it, so you get a stream rather than a blob; stone and plant are things you build with, so they go down once where you put them. Press Play to start time moving, or Step to advance it a single tick. On a phone, the Draw / Pan toggle decides what a drag does — Draw paints, Pan scrolls the box when you have zoomed in past the edge of the screen.

What each material does

• Sand falls straight down, and slides diagonally if it cannot. It never moves sideways, which is the only reason a pile of it is a cone rather than a puddle.
• Water does everything sand does and then moves sideways as well, so it keeps going until it is level or it runs into a wall.
• Oil is lighter than water, so it floats on top of it, and unlike water it burns.
• Stone never moves and acid cannot eat it. It is what you build containers, funnels and ledges out of.
• Plant never moves either, but it creeps into any water it touches, one cell a tick, turning the water into more plant. It burns easily.
• Fire rises, lights anything flammable next to it, and burns out after twenty-four ticks into smoke. Water puts it out.
• Acid sinks through water, eats one neighbouring cell per tick and is spent doing it — so getting through a thick wall takes a lot of acid, and stone stops it outright.

Two more appear without ever being painted. Smoke is what fire and acid leave behind; it rises and fades. Steam is what happens where fire meets water: both cells boil away, the steam rises, and after about forty ticks it condenses back into water and rains down again. Nothing is lost in that exchange — drop a fire into a pond and count the cells afterwards and the pond is exactly as full as it was, plus the one cell the fire itself became.

Why the order things are updated in matters

The box is 192 cells across and 128 down — 24,576 of them — and every tick walks the whole grid. Which order it walks in is not an implementation detail, it is part of the rules, and getting it wrong produces sand that behaves visibly wrongly.

This box sweeps from the bottom row upwards. Sweep downwards instead and a column of falling sand stops falling as a column: the top grain is looked at first, finds the grain below it still in place, and stays put, so only the very bottom grain of a stream moves each tick and the stream stretches into a dotted line. Each cell also carries a mark saying whether this tick has already dealt with it, which is what stops a grain being carried down the board several rows at once.

Within a row, the direction alternates — left to right on one tick, right to left on the next. That matters for a grain landing exactly on top of a one-cell peak, where both ways down are equally free and nothing but the sweep order decides. Without the alternation every grain that ever lands on that peak goes the same way, and a pillar sheds its whole pile to one side.

When the box stops

One tick in which nothing at all moved is enough to know nothing ever will, so the run halts there rather than redrawing a finished pile sixty times a second. A box of sand gets there; the hourglass takes about 1,600 ticks to empty and then stops.

A box with water in it usually does not, and that is a real trade rather than a bug. Water levels itself by shuffling sideways along its own surface, and a surface with a ragged top row looks exactly like a mound that ought to spread out — from a single cell's point of view they are the same picture. So either water levels and the surface never quite stops moving, or it stops and mounds of water stay mounds. This box chooses levelling.

Sharing a box you built

Copy link puts the whole box in the address, so a container you built can be sent to somebody exactly as it stands. Nothing is uploaded — the box travels in the part of the address after the #, which browsers never send to a server. Only the materials travel: a fire arrives with a full life ahead of it rather than the four ticks it had left, because what you are sharing is the scene, not the moment.