Rotate pipes by quarter-turns. You cannot move a tile, replace its shape, add pipe, or remove pipe.
Every pipe opening must meet a matching opening on the adjacent cell above, right, below, or left. No opening may face the board edge or an empty cell.
All non-empty pipe tiles must be connected together through matching openings.
The completed network must not contain a closed loop. Branches, tees, and crosses are allowed.
Empty cells stay empty and do not need to connect to anything.
You solve the puzzle only when all of those conditions hold simultaneously. A board with no leaks but two separate networks is incomplete; a fully connected board containing a loop is invalid.
No opening may point off the board, so every tile on an edge has fewer legal turns than one in the middle — and the corners fewest of all.
Example: A tee on the top row cannot open north, so only one of its four turns is legal: its three arms must face east, south and west.
An elbow in a corner has two of its four sides against the edge, and an elbow opens two adjacent sides. Exactly one turn points both arms inward.
Example: An elbow in the top-left corner must open east and south. Set it and never touch it again.
A blank cell holds no pipe, so an opening facing it leaks exactly as if it faced the board edge. Every empty cell shrinks its neighbours’ options the same way an edge does.
Example: A straight pipe with an empty cell directly above it cannot lie north–south, so it must lie east–west.
An end pipe has a single opening, so once you know which side it must use, its neighbour is committed to answering on the facing side.
Example: An end on the left edge beside an empty cell below it can only point east or north; if the cell above is also empty, east is forced — and the tile to its east must open west.
Both have two openings, but a straight uses opposite sides and an elbow uses adjacent ones. Once you know one side a two-way tile must use, its shape decides the other.
Example: A two-way tile that must open north is a straight if it also opens south, and an elbow if it opens east or west. Look at which neighbours can answer.
A tile’s shape fixes exactly how many edges it ends up with: one for an end, two for a straight or elbow, three for a tee, four for a cross. Count the sides already ruled out.
Example: A cross needs all four neighbours to answer, so a cross can never sit on an edge or beside an empty cell — if you see one there, an earlier deduction was wrong.
The finished network is a tree: every tile connected, and never a route back to where you started. Any turn that would join two ends of the same run is wrong.
Example: Two arms of the same connected section face each other across one cell. Turning that cell to join them would close a loop, so it must connect somewhere else.
A dry pipe is grey, water fills the section holding the source, and a sealed-off section turns red. All three are facts about the board you are looking at, not hints about the answer.
Example: A section that turns red is sealed off — it has no leaks left, so it can never grow to reach the rest. Undo back to where it closed and route it differently.