How to play Prism Sort

Pour the balls until every tube holds one colour. Each board carries a proven minimum, and tells you the fewest moves you can still finish in.

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Prism Sort is a ball sorting puzzle, and it belongs to the family that includes water sort and test tube puzzles. The rules take one line. Everything interesting comes from what the game can tell you about them, so this page covers the exact setup, what the numbers in the header mean, and how a board can end.

The setup

Every board holds four balls of each colour and two spare tubes, and every tube holds four balls. An Easy board has four colours and six tubes; Medium has six and eight; Hard has eight and ten; Expert has ten and twelve. The two spare tubes are the same on every board and at every size. They are not a difficulty setting — they are what makes the puzzle work, because without somewhere to put a ball down almost no deal could be untangled at all.

The balls start shuffled and dealt four to a tube, filling every tube except the two spares. Nothing is hidden. Everything you need to solve the board is visible from the first move, which is the point: this is a puzzle of arrangement, not of discovery.

What a pour does

Picking up a tube takes the whole run of one colour sitting on top of it — if the top three balls are green, you are holding three green balls, not one. Pouring puts that run onto another tube, and it is legal in exactly two cases: the target tube is empty, or the ball on top of the target is the same colour as the run and the tube is not full.

If the run does not fit, as much of it as fits goes across and the rest stays behind. That is worth watching for, because splitting a run costs you a move you will have to spend again later, and it is the single most common way a solve drifts past par.

One pour is one move, whether it carries one ball or four. Carrying four for the price of one is where good solves come from.

One move is not available: picking up a tube that is already a single colour and moving it to an empty tube. It would leave the board in exactly the same state with two tubes swapped, so the game does not offer it.

Par, and why it is not an estimate

Before a board reaches you it is solved. A search proves the shortest sequence of pours that finishes the deal, and that number becomes par. It is a minimum in the strict sense: if par says 24, then 23 is impossible, and the search has examined enough of the position to say so rather than merely failing to find one.

So par cannot be beaten, and matching it is a real result rather than a participation mark. Most solvers land a handful of moves over on a large board, and closing that gap is the game.

Fewest, the number beside par

Par describes the deal. Fewest describes you. After every pour the game solves the board again from your current position and shows the shortest finish still available, counted from your first move — so it begins equal to par, and rises by one each time you spend a move that did nothing.

This is the part most puzzles of this kind leave out, and it changes how the game is played. You do not reach the end wondering where the extra moves went. You see the moment it happens, and undo is free.

How a board can end

There are three endings. You sort every tube, which is a win. You run the board into a position with no legal pour at all, which is a dead stop. Or — the one that catches people — you reach a position that still has legal moves but can no longer be finished, because a colour has been buried under something that can never be lifted off it.

The third case is why Fewest also reports dead ends. When the board can no longer be sorted, the readout turns and says so, rather than letting you play another twenty moves before the truth arrives. Undo steps back one pour; reset returns the board to the deal with the counter at zero.

Hints and the daily board

A hint searches from wherever you actually are, not from the opening, so it stays useful however far you have wandered. It reports the best pour available now and how many moves remain from here.

One board a day, the same for everybody. The week ramps: Monday and Tuesday are easy, Wednesday and Thursday medium, Friday and Saturday hard, and Sunday is an expert board. Finishing extends your streak, solves that land exactly on par are counted separately, and the share text carries your moves against par without giving the board away.

Controls

Tap or click a tube to lift its top run of colour, then tap another tube to pour. Tap the same tube again to put it back.

Keyboard

Number keys pick a tube, Arrow keys move the cursor and Space pours.
Z undoes, H asks for a hint, R resets.

Strategy tips

Never split a run. If three greens are sitting on top and the target only has room for two, that pour costs you a move you will have to spend again — find somewhere the whole run fits, or move something else first.
Spend the empty tubes late. They are the only place an awkward run can go, and a board with both spares full has almost no legal moves left. Pouring onto a matching colour is nearly always better than parking on an empty tube.
Before you park a ball, know what it uncovers. Buffering is only worth a move if the ball underneath can then go home.
Look for colours already sitting on a tube floor. A colour resting on the bottom of a tube has a home to build on. A colour with no floor anywhere needs an empty tube before it can be gathered at all, and that is an extra move you can see coming.
Finish a tube the moment you can. A completed tube is out of the puzzle for good and stops competing for space.
Watch Fewest, not just Moves. While it matches par you are on a perfect line; the pour that pushes it up is the one worth undoing immediately, while you still remember what you were trying to do.
If Fewest reads a dash, stop. The board can no longer be sorted and no amount of further play will recover it — undo back to where it went wrong, or reset.
On a board you have finished well over par, reset and play it again straight away. The second run is where par is usually found, because you already know which colour was the awkward one.