diff --git a/packages/ludic.physics/shapes.ludic b/packages/ludic.physics/shapes.ludic
index b8ed83bd..f0f42f34 100644
--- a/packages/ludic.physics/shapes.ludic
+++ b/packages/ludic.physics/shapes.ludic
@@ -6,12 +6,15 @@ export function phys_open(physics_st: mut PhysicsState, max_bodies: int, threads
   phys_close(physics_st)
   ph_buffers(physics_st)
   physics_st.ph_world = jph_world_new(max_bodies, threads)
+  physics_st.ph_gen += 1
   physics_st.ph_acc = 0.0
   physics_st.ph_steps = 0
   return physics_st.ph_world != null
 }
 
 export function phys_is_open(physics_st: PhysicsState) -> bool { return physics_st.ph_world != null }
+# which world this is: a body, shape or walker id kept from an older one means nothing now
+export function phys_generation(physics_st: PhysicsState) -> int { return physics_st.ph_gen }
 
 # everything let go: the world, every body in it and every shape (a world swap, a quit)
 export function phys_close(physics_st: mut PhysicsState) -> void {
diff --git a/packages/ludic.physics/state.ludic b/packages/ludic.physics/state.ludic
index 9ec38552..3ee35b70 100644
--- a/packages/ludic.physics/state.ludic
+++ b/packages/ludic.physics/state.ludic
@@ -42,6 +42,7 @@ export state PhysicsState {
   ph_max_steps: int = 4           # a slow frame catches up this far and no further
   ph_acc: float = 0.0
   ph_steps: int = 0
+  ph_gen: int = 0                 # how many worlds have been opened: an id from an older one is stale
   ph_hard: float = 2.0            # metres a second: a contact under this is not a fact
   ph_floats: []int = null         # buoyant bodies' ids
   ph_float_k: []float = null      # and theirs: buoyancy, linear drag, angular drag, wet