Step 4 of 6

One tree, one tent

One tree, exactly one tent. When a tree is down to one open neighbor, that tent is forced.

The rule of thumb: A tree with a single open neighbor takes its tent there; a tree with its tent grasses the rest.

Each tree takes exactly one tent, on one of its four neighboring clearings — no more, no less. Two readings fall out of that. A tree with a single open neighbor left takes its tent there, immediately. And a tree that already has its tent turns every other neighbor to grass.

Edge trees are the kindest: three neighbors to begin with, two in a corner, so they run out of options first. These practice boards can’t be finished without the pairing — take this rule away and every other technique together stalls with trees still waiting.

Where to look: Trees whose neighbors are nearly all grass already. Count a tree’s open neighbors the way you count a line.

Worked example

A real position from a board that needs One tree, one tent. Purple: what the reasoning rests on. Dashed: the cells in question. Green: what it proves.

The tree at row 3, column 5 (purple) has exactly one open neighbor left. The dashed cell is that neighbor.

The tree at row 3, column 5 has only one open neighbor — its tent goes there.

Practice it ↓