Tents

Start with the cells no tree can reach

Before you place a single tent, sweep the board for cells that do not touch a tree on any side. A tent must sit directly beside its tree (left, right, above, or below), so a cell with no tree next to it can never hold a tent. Mark every one of those cells as grass right away. On most boards this clears a surprising amount of ground in the first minute, and every grass mark makes the numbers around the grid easier to use. Do the zeros next. A 0 beside a row or column means no tents anywhere in that line, so the whole line is grass. Between the no-tree cells and the zero lines, a fresh puzzle usually loses a third of its empty space before you have made one real decision.

Then hunt the forced moves

Now compare each edge number with the open cells left in its line. Two checks do most of the work. First, if the number matches the count of open cells exactly, every one of those cells is a tent. Fill them all. Second, once a line's number is already met by placed tents, everything else in that line is grass. Here is a worked case. Say a row is marked 3, and after your first sweep it has exactly three open cells left. All three are tents. Place them, then apply the other big rule: no two tents may touch, not even at a corner. Each new tent clears the eight cells around it, and those cleared cells reach into the neighboring rows and columns, which often completes their counts too. One forced row can unravel a quarter of the board. Trees pinned against a wall are the other easy win. A tree in a corner has at most two cells beside it, and if one is already grass, its tent is placed for free.

The pairing trap

The mistake that quietly breaks boards is assuming a tent belongs to the tree you had in mind. The pairing is strictly one to one, so tents always equal trees in number, but a tent standing between two trees could pair with either of them. Count pairs, not just placements. Picture two trees in a row with one empty cell between them. That cell touches both trees, yet it can hold only one tent, so one of those trees must find its tent on its far side. If that far side is already grass, the tent between them is claimed by the tree with no other option, and the second tree's tent is forced elsewhere. Chains of trees standing in a line work the same way, like dominoes: each tree-and-tent pair takes up space the next pair cannot use. When a row count suddenly looks impossible, the cause is usually a tent placed earlier that silently stole the only spot a nearby tree had left.

• Mark grass generously. Proving where tents cannot go is half the puzzle.
• After every tent, clear its eight neighbors before doing anything else.
• Recheck a line the moment its count is met. The leftover open cells there are grass.
• When stuck, pick a tree with two options and test each one against the row and column counts. One choice usually breaks something fast.

A modern pencil classic

Tents, often listed as Tents and Trees, comes from the world of magazine logic puzzles and spread widely once computer versions appeared, including one in Simon Tatham's well-known free puzzle collection. It belongs to the same family as Battleships and Star Battle: no arithmetic, just placement rules and counts along the edges. A well-made board never needs a guess. Every step is forced if you look in the right place, which is what makes the puzzle so pleasant to finish. Inkplay's version is timed, so the goal is to run the forced moves quickly: sweep the dead cells, clear the zero lines, and let the counts do the thinking. The click cycle is simple. One click pitches a tent, a second click marks grass, and a third clears the cell back to blank. You are done when every tree has its one tent, every number matches its line, and no two tents touch anywhere on the board.