Every sliding block puzzle obeys one law: pieces slide into empty space, and nothing rotates, jumps, or lifts off the board. Under that law live two families that play nothing alike. Knowing which one you are holding is half the strategy.
Family one: identical tiles, sort them
The sliding tile puzzle family. All pieces are 1×1, one cell is empty, and the goal is to arrange the tiles in order — numbers ascending, or a picture reassembled.
| Puzzle | Board | Try it |
|---|---|---|
| 8 puzzle | 3×3 | ten-minute introduction |
| 15 puzzle | 4×4 | the canonical one |
| 24 puzzle | 5×5 | a longer sit-down |
| 35 puzzle | 6×6 | patience training |
This family has a known, learnable method — solve the top row, then the left column, recurse into the remaining smaller board, finish the last 3×2 by rotation. The full walkthrough is here. It also has a famous trap: exactly half of all arrangements are unsolvable, which is why honest apps shuffle by walking backwards from the goal.
Family two: mixed blocks, extract one
Here the pieces have different sizes — 2×2, 1×2, 1×1 — and the goal is not order but escape: manoeuvre one designated block to the exit.
Klotski is the classic: a 4×5 board, a 2×2 hero block, and an escort of guards and pawns to untangle. The standard opening needs at least 81 moves. China knows it as Huarong Dao, France as L'âne rouge; the full story is in our Klotski guide.
Rush Hour is the modern commercial star: cars (1×2) and trucks (1×3) locked in a 6×6 grid, each able to slide only along its own axis, and a red car that must reach the exit. The axis restriction changes the feel — less rotation-dance than Klotski, more logistics.
Dad's Puzzle / Pennant Puzzle — early-20th-century American ancestors of Klotski with slightly different block mixes. Collectors' items now, but the same game.
There is no universal recipe for this family the way there is for tiles. What transfers: watch the empty cells (they are the real moving pieces), move the hero block rarely, and treat repeated shuffling of the small blocks as progress, not circling — solutions here genuinely do look like going backwards half the time.
Which is harder?
For a human: mixed-block puzzles feel harder per move but end sooner; tile puzzles are individually easy moves in much longer solutions.
For a computer, the answer is precise and surprising. A specific Klotski board is trivial — tens of thousands of positions, breadth-first search, optimal answer in milliseconds. A specific 15 puzzle is heavy — ten trillion states, needing IDA* with good heuristics. But generalised sliding block puzzles — any board, any blocks — are PSPACE-complete: proven (by Hearn and Demaine's nondeterministic constraint logic) to be as hard as any puzzle whose positions fit in memory. The innocent-looking family is, in the limit, the hardest kind of puzzle known.
Wooden, plastic, or on a screen
Physical sliding block puzzles are having a quiet renaissance — wooden Klotski and Rush Hour sets make good desk objects and better gifts, and the tactile click of a well-made slide is genuinely pleasant. The trade-off: no undo, no shuffle, and nobody stops you from "solving" a wooden 15 puzzle by prying the tiles out (the 1880 solution to Sam Loyd's unsolvable board).
On a screen you get the things wood cannot do: honest shuffles that are guaranteed solvable, a move counter, your own photo as the picture, and a solver for the day a board defeats you. Our app keeps the quiet of the wooden version — no ads, no streaks, no accounts — and adds exactly those four things.
Start here
If you are new to the genre: play the 8 puzzle first — it teaches the mechanic in ten minutes. Then the 15 puzzle for the real thing. Then Klotski, to discover how different the same mechanic can feel when the blocks stop being equal.