Free space is the real currency
Every board has a fixed amount of liquid and a fixed number of bottles, so the only variable under your control is where the empty units sit. Free space comes in two qualities:
- Universal space — units inside a completely empty bottle. Any colour can use them.
- Committed space — units above an existing colour. Only that colour can use them.
Nearly every bad move in Water Sort converts universal space into committed space for no return. When you look at a pour, ask: after this, can more colours use my remaining space, or fewer? A board with one empty bottle and six half-filled bottles looks busy but is often nearly dead, because almost all of its space is committed.
Fragment counting
A colour is a set of four units spread over one or more bottles. The number of places it appears is its fragment count. Think of it as work remaining:
| Fragments | Example layout | What it means |
|---|---|---|
| 1 | 4 units in one bottle | Finished. Ignore the bottle from now on. |
| 2 | 3 + 1, or 2 + 2 | One pour finishes it, if the top of the source is that colour. |
| 3 | 2 + 1 + 1 | Two pours at best, more if a fragment is buried. |
| 4 | 1 + 1 + 1 + 1 | The worst case. Usually the colour to attack last, not first. |
Two corollaries follow. First, prefer to finish colours that are already at two fragments — the payoff is one pour for one bottle freed. Second, never create a new fragment by a partial pour unless it directly unlocks something (see common mistakes).
How to compare two pours
When two moves both look reasonable, score them on four questions. The move that wins more questions is almost always correct:
- Does it empty a bottle? Emptying beats everything else, because it converts committed space back into universal space.
- Does it reduce a fragment count? Merging two pieces of one colour without leaving a remainder is pure progress.
- Does it expose a colour you need? A pour whose value is what it uncovers is usually better than a pour that only tidies tops.
- How many colours can use my space afterwards? If the answer drops from three to one, be suspicious even if the move looks tidy.
Worked comparison
Suppose bottle A is [green, green, pink], bottle B is [pink, pink] with two spaces, and bottle C is empty.
Move 1: pour A into B. The single pink lands on pink, bottle B is now three pink, and green is exposed at the top of A. Fragment count for pink drops from 2 to 1 group plus nothing wasted, universal space untouched.
Move 2: pour A into C. The pink goes into your empty bottle. Same colour exposed, but you have converted four units of universal space into a bottle that only accepts pink.
Both expose green. Move 1 wins on questions 1, 2 and 4, so it is clearly better — and the difference compounds over a full board.
Ordering: what to unlock first
Once you know which colours are close to finished, the question becomes sequencing. A useful rule: work on the colour whose remaining fragments are shallowest, and leave anything at the very bottom of a tall stack for when you have space to unload it.
The deepest unit on the board is the one that dictates how much space you eventually need. If a colour sits at the bottom of a bottle under three different colours, you will need somewhere for all three of those before you can reach it. Decide early where each of them is going. That is the difference between a plan and a sequence of hopeful taps — the routine for it is on the planning moves page.
Warning signs of a collapsing board
- All your space is committed to one colour. Two bottles each showing the same colour with one space left looks safe but only helps that colour.
- Three or more colours at fragment count three. Too much work remaining for the space you hold.
- Every top colour is different. With no empty bottle, that is a deadlock; with one empty bottle, it is one pour away from a deadlock.
- You are pouring to make the board look neater. Tidiness is not progress — fragment counts and free space are.
If you already see these signs, read solving difficult levels for the recovery order.
How to practise this
Open levels 11 to 20 in the browser game — five and six colours with two spare bottles. On each board, before your first pour, say out loud which colour has the fewest fragments and which unit is buried deepest. Then play, and use the hint only after you have decided your own move, so you can compare the two. The hint comes from a real solver, so a disagreement is always worth understanding.