Step 1 of 8
Only one cell can light it
A dark room with exactly one cell that could ever light it is a free torch.
The rule of thumb: A dark room with a single cell that could light it gives that cell its torch. Scan every dark room.
Every dark room needs a torch somewhere in its row or column that can reach it. Pillars chop those lines into short stretches, and sometimes a stretch narrows to a single candidate. That cell holds a torch: if it didn’t, the room would stay dark forever.
Make it a habit: after every torch you place, scan the dark rooms and count their candidates. The ones down to a single candidate are free torches, and each one you place narrows the rooms still dark. These practice boards can’t be finished without the scan — every other rule together stalls with rooms left dark.
Where to look: Dark rooms in short stretches: beside pillars, along edges, in corners. Count candidates, not torches.
Worked example
A real position from a board that needs Only one cell can light it. Purple: what the reasoning rests on. Dashed: the cells in question. Green: what it proves.
That’s the scanner’s mindset. Next: the pillar that tells you where torches can’t go. Step 2: "0" means mark them all →