Counting seven ghosts around a 7 is hard. But the window has eight cells, so flipped it is just one × — the yellow cell. Windows hold 8, 5 or 3 cells, so a 7, an edge 4 and a corner 2 all become the same flipped puzzle: find one ×. Past half the window, flip it — counting the complement, a classic math move.

In the app, one new tip arrives for each day you play.

▶ Try 50 rounds in your browser A free demo, no install. All 2,000 rounds are in the app.