diff --git a/lectures/myst.yml b/lectures/myst.yml
index 8ed2593..49fafa4 100644
--- a/lectures/myst.yml
+++ b/lectures/myst.yml
@@ -35,6 +35,7 @@ project:
       #       - localStorage
   toc:
     - file: intro.md
+    - file: probe_nesting
     - title: Economic Data 
       children:
         - file: long_run_growth
diff --git a/lectures/probe_nesting.md b/lectures/probe_nesting.md
new file mode 100644
index 0000000..765b390
--- /dev/null
+++ b/lectures/probe_nesting.md
@@ -0,0 +1,88 @@
+---
+jupytext:
+  text_representation:
+    extension: .md
+    format_name: myst
+    format_version: 0.13
+    jupytext_version: 1.16.7
+kernelspec:
+  display_name: Python 3 (ipykernel)
+  language: python
+  name: python3
+title: "PROBE — which code cells actually execute?"
+---
+
+# Probe: which code cells actually execute under thebe-lite?
+
+Temporary diagnostic page. **Not for merge.** Run the cells top to bottom, then run
+the final REPORT cell — it names every cell that actually executed, so a cell that
+silently did nothing is visible rather than ambiguous.
+
+## Setup (top level — control, must work)
+
+```{code-cell} ipython3
+ran = []
+ran.append("A_toplevel_setup")
+print("A ok")
+```
+
+## B — top level, `%%file` magic (tests the magic independently of nesting)
+
+```{code-cell} ipython3
+%%file probe_b.txt
+hello-from-B
+```
+
+```{code-cell} ipython3
+import os
+ran.append("C_toplevel_after_magic")
+print("C: probe_b.txt exists?", os.path.exists("probe_b.txt"))
+if os.path.exists("probe_b.txt"):
+    print("C: contents =", open("probe_b.txt").read().strip())
+```
+
+## D — inside a GATED exercise
+
+```{exercise-start}
+:label: probe_ex
+```
+Prose inside the gated exercise.
+
+```{code-cell} ipython3
+ran.append("D_in_gated_exercise")
+print("D ok")
+```
+```{exercise-end}
+```
+
+## E — inside a GATED solution
+
+```{solution-start} probe_ex
+:class: dropdown
+```
+```{code-cell} ipython3
+ran.append("E_in_gated_solution")
+print("E ok")
+```
+```{solution-end}
+```
+
+## F — inside a plain admonition
+
+````{note}
+```{code-cell} ipython3
+ran.append("F_in_note")
+print("F ok")
+```
+````
+
+## REPORT (top level)
+
+```{code-cell} ipython3
+expected = ["A_toplevel_setup", "C_toplevel_after_magic",
+            "D_in_gated_exercise", "E_in_gated_solution", "F_in_note"]
+print("EXECUTED:", ran)
+print("MISSING :", [e for e in expected if e not in ran])
+import os
+print("%%file worked at top level:", os.path.exists("probe_b.txt"))
+```