diff --git a/Behaviors.md b/Behaviors.md
index bfc83e0..6e43f1e 100644
--- a/Behaviors.md
+++ b/Behaviors.md
@@ -128,7 +128,9 @@ shown on the node and editable in the inspector.
   `Apply force`, `Apply impulse`, `Set velocity`, `Set angular velocity`, `Set gravity` (see [Physics](Physics)),
   `Raycast` (what a ray meets first on the layers it looks at: object, collider, point, normal, distance — see [Colliders](Colliders#raycast)),
   `Raycast all` (everything on that line instead, as parallel lists sorted by distance, with a `count` and a `max hits` — see [Colliders](Colliders#raycast-all)),
-  `Ignore one-way` (fall through one-way colliders and tiles for some seconds — see [Colliders](Colliders#one-way)). A node type is one file exporting a `NodeDef`
+  `Ignore one-way` (fall through one-way colliders and tiles for some seconds — see [Colliders](Colliders#one-way)),
+  `Set tile` with the data nodes `Tile at`, `World to cell`, `Cell to world` and the constant `Tile`
+  (read and change the cells of a placed tilemap while the game runs — see [Tilemaps](Tilemaps#from-a-behavior)). A node type is one file exporting a `NodeDef`
   (ports, settings, `run`), listed in `nodes/index.ts`; palette, ports, validation and
   inspector fields are generated from it. A port may declare a `default` for its inline value
   (e.g. `Move.scale = 1`), shown on the node and in the inspector.
diff --git a/Development.md b/Development.md
index e25f367..94abdb2 100644
--- a/Development.md
+++ b/Development.md
@@ -14,7 +14,7 @@ npm run build        # writes dist/ and regenerates index.html
 ./build.sh           # the same under `set -o pipefail`, for callers that pipe the output
 php tools/make-user.php <name> <password> [admin|user]        # an account; every tool signs in (see Accounts)
 php tests/php/accounts.test.php                               # the API's account rules, no server needed (CI's gate job)
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1351 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1356 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result
diff --git a/Tilemaps.md b/Tilemaps.md
index 3939ae1..88d3e2a 100644
--- a/Tilemaps.md
+++ b/Tilemaps.md
@@ -146,6 +146,41 @@ tile is on the same step, so animated water does not run out of phase between tw
 Drawing is done with **one mesh per tile and layer** rather than one sprite per cell, so a map of a
 thousand cells costs as many draw calls as it has tiles on that layer.
 
+## From a behavior
+
+A behavior reads and changes the cells of a placed map while the game runs (#135). Four nodes do
+it, each taking the placed map as its `tilemap` input:
+
+- **`Tile at`** gives the tile in a cell (`tile`, empty for none) and `empty`. It reads the moment
+  it is asked, so a `Set tile` earlier in the same flow is already seen. A cell outside the grid or
+  on a layer the map does not have reads as empty.
+- **`Set tile`** puts a tile into a cell, or empties it when no tile is given. `done` says whether
+  it could: not on something that is no tilemap, not on a missing layer, not outside the grid, and
+  not with an asset that is no tile.
+- **`World to cell`** turns a world point into a cell, `inside` saying whether that cell is on the
+  grid. This is how a figure finds the cell it stands on. The map may be moved, turned, scaled and
+  mirrored.
+- **`Cell to world`** gives the middle of a cell in world units, the way back.
+
+A **`Tile`** node hands out one tile as a constant: something for `Set tile` to put down, or to
+compare what `Tile at` read with (`Equals`).
+
+Cells count in whole columns and rows, `0, 0` being the top-left corner, as in the panel. The
+layer is picked **by name** from the layers of all tilemaps in the project, since the node does not
+know which map it will be given. The index would move whenever the layers are reordered. *First
+layer* means the first layer of whatever map arrives; two layers of one name are the upper one.
+
+What a behavior sets **belongs to that one placement and to the run**:
+
+- The run keeps its changes per placed map, laid over the asset's cells. A second placement of the
+  same map is left alone.
+- The changed cell is drawn at once, the autotile edges of its neighbours follow, and its collision
+  is rebuilt: a `Move` later in the same frame already meets the new cell, and a falling body lands
+  on it.
+- **Nothing is written back.** After Stop the map is the one that was painted. The editor run is no
+  exception: the scene view shows the changed cells while the run lasts and the painted ones again
+  once it ends, and the project never sees them.
+
 ## Collision
 
 Whether a cell blocks is a property of the **tile**, not of the map or the layer: switch **Solid**