diff --git a/slime-exp/examples/rain.py b/slime-exp/examples/rain.py
new file mode 100644
index 0000000..f6833b4
--- /dev/null
+++ b/slime-exp/examples/rain.py
@@ -0,0 +1,26 @@
+from slime.grid import Grid
+from slime.rules import SimpleMapping, Movement, Combine
+
+grid = Grid(10, 10, wrap_y=True)
+grid[0, 0, "state"] = 1
+grid[1, 1, "state"] = 1
+grid[2, 0, "state"] = 1
+grid[3, 1, "state"] = 1
+grid[4, 0, "state"] = 1
+grid[5, 1, "state"] = 1
+grid[6, 0, "state"] = 1
+grid[7, 1, "state"] = 1
+grid[8, 0, "state"] = 1
+grid[9, 1, "state"] = 1
+
+# grid.rules.append(SimpleMapping("state", {0: 1, 1: 0}))
+# grid.rules.append(Movement("state", 0, 1))
+grid.rules.append(
+    Combine(SimpleMapping("state", {0: 1, 1: 0}), Movement("state", 0, 1))
+)
+
+while True:
+    # grid.render_text("state")
+    grid.render_text(lambda cell: {0: "|", 1: "-"}[cell["state"]])
+    input()
+    grid.step()
diff --git a/slime-exp/src/slime/grid.py b/slime-exp/src/slime/grid.py
new file mode 100644
index 0000000..2ce0060
--- /dev/null
+++ b/slime-exp/src/slime/grid.py
@@ -0,0 +1,118 @@
+from enum import Enum
+from collections import defaultdict
+from .rules import Rule
+
+
+class Connected(Enum):
+    FOUR_WAYS = 4
+    EIGHT_WAYS = 8
+
+
+class Grid:
+    def __init__(
+        self,
+        width,
+        height,
+        *,
+        connected=Connected.FOUR_WAYS,
+        wrap_x=False,
+        wrap_y=False,
+        default_value=dict,
+    ):
+        self.width = width
+        self.height = height
+        self.connected = connected
+        self.wrap_x = wrap_x
+        self.wrap_y = wrap_y
+        self.grid = defaultdict(default_value)
+        self.rules = []
+
+    def neighbors(self, x, y):
+        x1 = x - 1
+        x2 = x + 1
+        y1 = y - 1
+        y2 = y + 1
+        if x1 < 0:
+            x1 = self.width - 1 if self.wrap_x else None
+        if x2 >= self.width:
+            x2 = 0 if self.wrap_x else None
+        if y1 < 0:
+            y1 = self.height - 1 if self.wrap_y else None
+        if y2 >= self.height:
+            y2 = 0 if self.wrap_y else None
+
+        if x1 is not None:
+            yield (x1, y)
+        if x2 is not None:
+            yield (x2, y)
+        if y1 is not None:
+            yield (x, y1)
+        if y2 is not None:
+            yield (x, y2)
+        if self.connected == Connected.EIGHT_WAYS:
+            if x1 is not None and y1 is not None:
+                yield (x1, y1)
+            if x1 is not None and y2 is not None:
+                yield (x1, y2)
+            if x2 is not None and y1 is not None:
+                yield (x2, y1)
+            if x2 is not None and y2 is not None:
+                yield (x2, y2)
+
+    def __getitem__(self, item):
+        x, y, prop = item
+        if self.wrap_x:
+            x %= self.width
+        if self.wrap_y:
+            y %= self.height
+        if x < 0 or x >= self.width or y < 0 or y >= self.height:
+            raise IndexError
+        return self.grid[item][prop]
+
+    def __setitem__(self, item, value):
+        x, y, prop = item
+        if self.wrap_x:
+            x %= self.width
+        if self.wrap_y:
+            y %= self.height
+        if x < 0 or x >= self.width or y < 0 or y >= self.height:
+            raise IndexError
+        self.grid[x, y][prop] = value
+
+    def __iter__(self):
+        return iter(self.grid.items())
+
+    def render_text(self, prop_or_func):
+        if callable(prop_or_func):
+            func = prop_or_func
+        else:
+            func = lambda x: x[prop_or_func]
+        for y in range(self.height):
+            for x in range(self.width):
+                if (x, y) in self.grid:
+                    print(func(self.grid[(x, y)]), end="")
+                else:
+                    print(" ", end="")
+            print()
+
+    def add_rule(self, rule):
+        self.rules.append(rule)
+
+    def step(self):
+        new_grid = defaultdict(dict)
+        for (x, y), cell in self.grid.items():
+            for rule in self.rules:
+                for update in rule.step(x, y, cell, self):
+                    # TODO: two rules shouldn't be able to update the same cell
+                    if update.x < 0 or update.x >= self.width:
+                        if not self.wrap_x:
+                            continue
+                        else:
+                            update.x %= self.width
+                    if update.y < 0 or update.y >= self.height:
+                        if not self.wrap_y:
+                            continue
+                        else:
+                            update.y %= self.height
+                    new_grid[update.x, update.y].update(update.data)
+        self.grid = new_grid
diff --git a/slime-exp/src/slime/rules.py b/slime-exp/src/slime/rules.py
new file mode 100644
index 0000000..4551f78
--- /dev/null
+++ b/slime-exp/src/slime/rules.py
@@ -0,0 +1,51 @@
+from abc import ABC, abstractmethod
+from dataclasses import dataclass
+from typing import Generator
+
+
+@dataclass
+class CellUpdate:
+    x: int
+    y: int
+    data: dict
+
+
+class Rule(ABC):
+    @abstractmethod
+    def step(
+        self, x: int, y: int, cell: dict, grid: "Grid"
+    ) -> Generator[CellUpdate, None, None]:
+        pass
+
+
+class SimpleMapping(Rule):
+    def __init__(self, prop, mapping):
+        self.mapping = mapping
+        self.prop = prop
+
+    def step(self, x, y, cell, grid):
+        yield CellUpdate(x, y, {self.prop: self.mapping.get(cell.get(self.prop))})
+
+
+class Movement(Rule):
+    def __init__(self, prop, dx, dy):
+        self.prop = prop
+        self.dx = dx
+        self.dy = dy
+
+    def step(self, x, y, cell, grid):
+        yield CellUpdate(x + self.dx, y + self.dy, cell)
+
+
+class Combine(Rule):
+    def __init__(self, *rules):
+        self.rules = rules
+
+    def step(self, x, y, cell, grid):
+        updates = list(self.rules[0].step(x, y, cell, grid))
+        for rule in self.rules[1:]:
+            next_updates = []
+            for upd in updates:
+                next_updates.extend(rule.step(upd.x, upd.y, upd.data, grid))
+            updates = next_updates
+        yield from updates