diff --git a/packages/ludic.physics/index.ludic b/packages/ludic.physics/index.ludic
index d269641f..9fd169bc 100644
--- a/packages/ludic.physics/index.ludic
+++ b/packages/ludic.physics/index.ludic
@@ -5,6 +5,7 @@ numbers float
 import "ludic.base"
 import "native.ludic"
 import "state.ludic"
+import "shape_ids.ludic"
 import "shapes.ludic"
 import "bodies.ludic"
 import "queries.ludic"
diff --git a/packages/ludic.physics/shape_ids.ludic b/packages/ludic.physics/shape_ids.ludic
new file mode 100644
index 00000000..199bc482
--- /dev/null
+++ b/packages/ludic.physics/shape_ids.ludic
@@ -0,0 +1,33 @@
+# shape_ids.ludic - a shape id is its place in ph_shapes. A shape let go leaves its place to the next
+# one made, so a ground that comes and goes by chunk grows nothing
+
+function ph_keep(physics_st: mut PhysicsState, s: pointer) -> int {
+  if s == null { return -1 }
+  let n = len(physics_st.ph_spare)
+  if n > 0 {
+    let id = physics_st.ph_spare[n - 1]
+    List.pop(physics_st.ph_spare)
+    physics_st.ph_shapes[id] = s
+    return id
+  }
+  push(physics_st.ph_shapes, s)
+  return len(physics_st.ph_shapes) - 1
+}
+
+function ph_shape(physics_st: PhysicsState, id: int) -> pointer {
+  if physics_st.ph_shapes == null or id < 0 or id >= len(physics_st.ph_shapes) { return null }
+  return physics_st.ph_shapes[id]
+}
+
+# a shape let go: a body made with it keeps its own hold until it is removed, and the id may be
+# handed to the next shape made
+export function phys_shape_free(physics_st: mut PhysicsState, id: int) -> void {
+  let s = ph_shape(physics_st, id)
+  if s == null { return }
+  jph_shape_free(s)
+  physics_st.ph_shapes[id] = null
+  push(physics_st.ph_spare, id)
+}
+
+# how many shapes are held: a ground that comes and goes must hold this steady
+export function phys_shapes(physics_st: PhysicsState) -> int { return len(physics_st.ph_shapes) - len(physics_st.ph_spare) }
diff --git a/packages/ludic.physics/shapes.ludic b/packages/ludic.physics/shapes.ludic
index ead672d0..3ec7a04d 100644
--- a/packages/ludic.physics/shapes.ludic
+++ b/packages/ludic.physics/shapes.ludic
@@ -32,23 +32,12 @@ export function phys_close(physics_st: mut PhysicsState) -> void {
     }
   }
   physics_st.ph_shapes = new []pointer
+  List.clear(physics_st.ph_spare)
   List.clear(physics_st.ph_floats)
   List.clear(physics_st.ph_float_k)
   if physics_st.ph_facts != null { q_clear(physics_st.ph_facts) }
 }
 
-function ph_keep(physics_st: mut PhysicsState, s: pointer) -> int {
-  if s == null { return -1 }
-  ph_buffers()
-  push(physics_st.ph_shapes, s)
-  return len(physics_st.ph_shapes) - 1
-}
-
-function ph_shape(physics_st: PhysicsState, id: int) -> pointer {
-  if physics_st.ph_shapes == null or id < 0 or id >= len(physics_st.ph_shapes) { return null }
-  return physics_st.ph_shapes[id]
-}
-
 # a crate: half extents in metres
 export function phys_box(physics_st: mut PhysicsState, hx: float, hy: float, hz: float) -> int { return ph_keep(physics_st, jph_shape_box(hx, hy, hz)) }
 export function phys_sphere(physics_st: mut PhysicsState, r: float) -> int { return ph_keep(physics_st, jph_shape_sphere(r)) }
diff --git a/packages/ludic.physics/state.ludic b/packages/ludic.physics/state.ludic
index b99bb926..dd5cf4cc 100644
--- a/packages/ludic.physics/state.ludic
+++ b/packages/ludic.physics/state.ludic
@@ -37,6 +37,7 @@ export const PH_CAP: int = 512        # contacts drained a step, queries' buffer
 export state PhysicsState {
   ph_world: pointer = null
   ph_shapes: []pointer = new []pointer     # a shape id is its place here; null once freed
+  ph_spare: []int = new []int              # the freed places, handed out again (shape_ids.ludic)
   ph_step: float = 0.016666668    # seconds a step
   ph_sub: int = 1                 # collision steps in each
   ph_max_steps: int = 4           # a slow frame catches up this far and no further
diff --git a/packages/ludic.physics/tests/shape_free_test.ludic b/packages/ludic.physics/tests/shape_free_test.ludic
new file mode 100644
index 00000000..11e2831c
--- /dev/null
+++ b/packages/ludic.physics/tests/shape_free_test.ludic
@@ -0,0 +1,48 @@
+# shape_free_test.ludic - a shape let go: its body stands until removed, its id goes to the next
+# shape made, and a ground that comes and goes a thousand times holds no more shapes than at once
+import "ludic.physics"
+import "ludic.base"
+program ShapeFreeTest {
+  numbers float
+  function down(physics_st: mut PhysicsState, x: float, z: float) -> PhysHit {
+    let h = new PhysHit
+    phys_ray(physics_st, x, 50.0, z, 0.0, -100.0, 0.0, PHYS_M_ALL, h)
+    return h
+  }
+  # a flat 33 x 33 field at height y, 2 m cells, its first sample at (ox, 0)
+  function field(physics_st: mut PhysicsState, h: []float, ox: float, y: float) -> int {
+    for i in 0 .. 33 * 33 { h[i] = y }
+    return phys_heightfield(physics_st, h, 33, ox, 0.0, 2.0)
+  }
+
+  test "a body outlives its shape's id, and the id is handed out again" (physics_st: mut PhysicsState) {
+    expect(phys_open(physics_st, 64, 1))
+    let h = floats(33 * 33)
+    let s = field(physics_st, h, 0.0, 3.0)
+    let b = phys_ground_add(physics_st, s)
+    phys_shape_free(physics_st, s)
+    phys_settle(physics_st)
+    expect(Math.abs(down(physics_st, 10.0, 10.0).y - 3.0) < 0.01)
+    expect_eq(field(physics_st, h, 100.0, 5.0), s)
+    phys_remove(physics_st, b)
+    expect(down(physics_st, 10.0, 10.0).body < 0)
+    phys_close(physics_st)
+  }
+
+  test "a thousand chunks in and out hold two shapes and grow nothing" (physics_st: mut PhysicsState) {
+    expect(phys_open(physics_st, 64, 1))
+    let h = floats(33 * 33)
+    let keep = phys_box(physics_st, 1.0, 1.0, 1.0)
+    for k in 0 .. 1000 {
+      let s = field(physics_st, h, float(k % 7) * 64.0, 1.0)
+      let b = phys_ground_add(physics_st, s)
+      phys_remove(physics_st, b)
+      phys_shape_free(physics_st, s)
+    }
+    expect_eq(phys_shapes(physics_st), 1)
+    expect(phys_box(physics_st, 1.0, 1.0, 1.0) != keep)
+    expect_eq(phys_shapes(physics_st), 2)
+    phys_close(physics_st)
+    expect_eq(phys_shapes(physics_st), 0)
+  }
+}