A fixed tile fixes its neighbours
Each tile you are sure of tells its neighbours what to do.
Two neighbouring tiles must agree: an arm meets an arm, and a side without an arm meets a side without one. So once a tile is fixed, its neighbours lose every turn that does not fit. Pin each tile you are sure of (right click or long press) and follow the chain.
How it works
The pinned tiles are certain. Look at the free tiles next to them.
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.
Turn the free tiles so that every lamp lights up.
Why it works
Most moves of a game are this rule. Pinning makes certainty visible: a pinned tile cannot be turned by a stray click, and you see at once where the chain can go on.