Two empty tubes can disappear quickly when every awkward ball gets its own parking place. SortStack becomes easier to reason about when you ask what a move exposes and what space it creates. Each tube holds up to four balls, and each normal move transfers just the top ball. A destination must be empty or have the same color on top, with room for another ball.
A completed tube changes the board
Four matching balls complete a tube. That tube locks, and the game adds a new empty tube to the right. This is why an early completion can be valuable even when it does not uncover a buried color immediately. It creates four more slots of workspace. On a narrow screen, scroll within the tube area to find the new tube.
Do not treat the four-ball tube as a pouring container: a run of three matching balls takes three individual moves to transfer. When playing with a move limit, count those transfers before starting a rearrangement. Emptying a tube in your imagination is easy; reaching that state within the remaining moves is the part to check.
Read this small position from bottom to top
| Tube | Bottom → top |
|---|---|
| A | 1 · 1 · 1 · 2 |
| B | 2 · 2 · 2 · 1 |
| C | 3 · 3 · 3 |
| D | 3 |
| E | Empty |
This is a constructed three-color position with twelve balls. The letters name tubes; the numbers name colors, matching the numbered balls used by the game. A list such as A = [1, 1, 1, 2] has 2 on top. Initially no tube is locked. Tube E is empty. The example uses the same capacity and movement rules as a normal round, but it is not an identified Daily puzzle or a randomly generated starting position.
- D → C. Move color 3 onto the three matching balls in C. C becomes complete and locks. D is now empty, and a new empty tube F appears. You have improved your working space with one move.
- B → E. Move the top color 1 from B into empty E. This exposes color 2 in B, leaving B = [2, 2, 2]. The transfer has a specific purpose: it prepares a matching destination for A’s top ball.
- A → B. Move color 2 onto B. B completes, locks and creates another empty tube. A is now [1, 1, 1].
- E → A. Return color 1 to its matching group. A completes. All three colors are sorted in four moves; the empty tubes need no filling.
Why the destination matters
At the start, B → C is illegal: color 1 cannot land on color 3. Moving D → E is legal, but merely parks a 3 in an empty tube; it does not expose a hidden ball or complete a group. D → C finishes that color and creates extra space immediately. This comparison explains a useful preference, not a proof that every position should begin with a completion.
After D → C, several empty tubes are available. Putting B’s top ball into D instead of E would also support the same plan. The important feature is the destination’s role, not its letter: it temporarily stores color 1 until A is ready. Avoid scattering the same color across many empties without a reason.
Plan backward from an exposed group
Find a tube with three matching balls and ask where its fourth ball is. If that ball is buried, identify exactly which top balls must move and where each can legally go. A plan is incomplete if it says “move these away” without naming available destinations. A full tube of a different color is not a spare slot, even if you intend to empty it later.
Free Play Easy uses three colors and five initial tubes without a move limit. Medium starts with five colors and seven tubes with 50 moves; Hard has seven colors and nine tubes with 40; Expert has ten colors and twelve tubes with 35. Relaxed removes the move limit. These settings change how much experimentation is comfortable. They do not promise that a hint gives a shortest solution.
Try a two-move explanation
Choose Easy or Relaxed for practice. Before selecting a source, finish this sentence: “I move this ball here so that my next move can…” If the second move still has no legal destination, rethink the first. Notice when a completion adds a tube, then reassess the board instead of continuing an old plan automatically. After solving, look for a parking move that could have been avoided; compare that one decision on your next round rather than trying to memorize a universal sequence.