diff --git a/demos/doc_demo.py b/demos/doc_demo.py
index 31090100..ea3b4587 100644
--- a/demos/doc_demo.py
+++ b/demos/doc_demo.py
@@ -3,6 +3,7 @@
 """
 from functools import partial
 from os import path, listdir
+import time
 
 from pywebio import start_server
 from pywebio.output import *
diff --git a/pywebio/output.py b/pywebio/output.py
index 9fa65481..794ff5a1 100644
--- a/pywebio/output.py
+++ b/pywebio/output.py
@@ -904,7 +904,8 @@ def put_loading(shape='border', color='dark', scope=Scope.Current, position=Outp
                 put_loading(shape=shape, color=color)
 
         ## ----
-        # Use as context manager
+        import time  # ..demo-only
+        # Use as context manager, the loading prompt will disappear automatically when the context block exits.
         with put_loading():
             time.sleep(3)  # Some time-consuming operations
             put_text("The answer of the universe is 42")