No loops

The network is a tree: one way between any two tiles.

Between any two tiles of the solution there is exactly one way along the pipes. So when two neighbouring tiles are already joined by a route around, the pipe between them must stay shut: opening it would close a loop.

How it works

Your turn

Four positions, each needing this lesson’s technique. The pinned tiles are given; turn the free ones until every lamp is lit. Click a tile to turn it, Ctrl+click to turn it back; right click or long press pins it.

Why it works

Loops are easy to miss, because each tile looks fine on its own. Follow the joined pipes with your eye: where a route comes back next to itself, the joint in between stays shut.