Mio Path / Guide
What is a one-line puzzle?
A one-line puzzle (also called a one-stroke, unicursal, or fill-the-grid puzzle) has a single rule: draw one continuous line that visits every tile in the grid exactly once. No crossing, no lifting, no tile left out.
The rule, and why it is hard
The grid is small and the rule is short, so the difficulty comes from the shape. Every tile you enter is a tile you cannot return to, which means a wrong early turn only shows up as a dead end many moves later.
The puzzle is a Hamiltonian path problem in disguise: find a route that visits every node once. Computers find it hard in general. People solve it by pattern, which is what makes it satisfying.
Extra tiles change the rule
Most one-line games add tiles that bend the basic rule. In Mio Path: an arrow forces your next step in its direction, ice makes the line slide until it hits a wall, a key must be collected before its door opens, and a portal jumps the line to its twin.
Each new tile is introduced on its own before it is combined with the others, so the rules stay learnable.
Tips that actually help
Start from the corners and edges. A corner tile has only two neighbours, so the line can only pass through it one way.
Look for tiles with a single free neighbour: those must be the start or the end of the path.
Do not force the middle. Solve the outline first and the interior usually falls into place.
If you are stuck, undo two or three moves instead of one. The mistake is rarely the last step.
Where to play
Mio Path, by OLYMPHEON, is a one-line puzzle with more than 2,500 hand-tuned levels, fully playable offline, in 34 languages. It is free on the App Store and Google Play.