Flow Fill

A six-by-six board of ground marks — circles, squares, triangles — and the marks repeat. Cells carrying the same mark and joined edge to edge form one basin, and water poured anywhere in a basin fills all of it. So the whole puzzle is a reading task: work out how many separate basins the board really holds, set one tap in each, and open the valve once. A corner join is not a join, which is where most of the pours go missing.

It runs entirely on your device: 0 network requests, works offline. No ads, no account, nothing to buy, and no betting, money or prizes of any kind.

1. Focus the board — click it, or press Tab until it is focused.
2. Move the cursor with the arrow keys, or the W, A, S and D keys.
3. Set or lift a tap with Enter or Space. Taps cost nothing and you can move them around as long as you like — nothing is spent until water moves.
4. Open the valve with F. Every tap pours at once, and each one fills the whole basin it is standing in. The valve costs one pour per tap, so two taps in the same basin spend two pours and fill one basin.
5. Lift every tap at once with Esc if you want to start the plan again.
6. Watch the two numbers: dry basins tells you how many are still waiting, and the pour count tells you what is left in the tank. Fill every basin before the tank empties.

How do you count the separate regions in a grid?

Pick an unvisited cell, then spread out from it to every neighbour above, below, left and right that carries the same mark, and keep spreading from each cell you reach until nothing new is added. That whole spread is one region. Cross it off, find the next unvisited cell, and repeat — the number of times you had to start over is the number of regions. Two rules do all the work. Diagonals do not count, so two patches meeting at a corner are two regions no matter how much they look like one. And the mark alone proves nothing: the same mark can appear in three unconnected places on the board, and each of those is its own region. That is the flood fill algorithm, and doing it by eye is exactly what this game asks for.

What is the fewest pours a board can be finished in?

Exactly the number of basins, and never fewer. One pour fills one basin and cannot reach into another, so every basin needs a pour of its own; one tap in each of them, opened in a single pull, shows that many is enough. The tank holds two more than that, which is your allowance for reading the board wrong — a third mistake ends the round.

Is every board actually fillable?

Yes, by construction. The basins are grown first, one cell at a time and only ever into a cell they already touch, so no basin is ever in two pieces. The marks are then painted on so that no two touching basins wear the same one, which is what makes the board honest: "same mark, edge to edge" recovers the true basins exactly, with nothing hidden. Because the marks are chosen to be as few as possible, some basins always share one — that is the reading task, not a trick.

Why did my pour do nothing?

Because it landed in a basin that already held water, or you set two taps in one basin without noticing they were joined. Look for a thin neck of the same mark running between two patches: one cell is enough to make them a single basin, and then the second tap is spent for nothing. Pulling the valve with no taps set also empties the tank, which is why the valve tells you what the next pull costs before you press it.

Do I need to see colour to play this?

No. Every ground mark is a shape — circle, triangle, square, diamond, cross, star — not a hue, and water is drawn as its own mark on the cell along with a tap ring where you have set one. Each cell announces its mark, whether it is wet and whether it holds a tap. The board is complete in greyscale and in forced-colours mode.

Is there any money, betting or in-app purchase in this?

None, and there never will be. No currency, nothing to buy, no extra pours for sale, and nothing is staked on the result.

Can I play it with a keyboard or a screen reader?

Both. Arrows move the cursor, Enter sets or lifts a tap, F opens the valve and Esc lifts every tap. The live region reads out how many basins are still dry, how many taps are set, what the next pull costs and what is under the cursor — which is precisely what the board shows and nothing more, since working out the basins is the game.

Does it work offline?

Yes. Once the page has loaded once, the whole game is on your device. No server is involved in play at any point, and there is no account to lose your progress behind.

Six by six, six basins, and a tank of eight pours. Boards are painted with at most four marks, so at least one pair of separate basins always shares a mark. Basins are grown rather than drawn, so sizes vary from a single cell to a dozen. The round also ends early when fewer pours remain than basins are dry, because at one basin per pour that position is already finished — the board says so rather than letting you keep pouring.

Grid Paint works the same connectivity from the other direction: there you build the patches instead of reading them. Light Grid is the other board here where one press changes a whole neighbourhood at once, and rewards the same habit of working out the effect before you commit to it.

Bookmark this page (Ctrl+D, or ⌘D on Mac) or install the app — it works offline the next time you need it.