How to play Herons.

1. Every pair of nests belongs to one pair of herons. Draw a flight path between the two nests of every pair. A path steps from one cell to the next, up and down or left and right, never diagonally, and it never runs beside itself: no cell of a path may touch another cell of the same path except the one before it and the one after it.

2. Paths never cross. Two different pairs never share a cell, so once a cell belongs to one path it is closed to every other.

3. Every cell of the marsh is used. No cell is left over when the marsh is finished; when every pair is joined and every cell is used, the game notices by itself.

Nothing lights up while you play. A cell given to the wrong path is not flagged, even a plain mistake; only Check judges, and only when you ask. Press it once and it says how many cells are wrong; press it again and it shows which, with a ring and a small cross, not colour alone.

On the phone, drag out of a nest and the path follows your finger; drag back along it and it rubs out. On a keyboard, arrows move, Enter starts and ends drawing, Delete clears, P pauses.

Every marsh opens with steps like these. Hint finds the next one for you, rings the cells it settles and says why; press it again to draw them in. Hints are counted and shown when you finish, never hidden.

• A nest with one way out. A nest in a corner, or with only one free side, must send its path that way.
• Only one way through. A cell with only one free side left has its path decided for it, because every cell of the marsh is used.
• No touching itself. A cell next to a path that is already there is closed to that same path, even when it looks free.
• Only one pair can reach it. When a corner of the marsh can only be reached by one pair, every cell of that corner belongs to it.