The crosses leave the left 1 only the three green cells — and all three sit inside the right window. The left ghost is somewhere in green, so it is also the right clue ghost. Then nobody is left for the yellow cells: all ×. If everything fits inside the smaller set, the outside is empty — the idea of set inclusion.

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.