diff --git a/core/lib/Drupal/Core/Cache/MemoryBackend.php b/core/lib/Drupal/Core/Cache/MemoryBackend.php
index 76bff41..2f24b76 100644
--- a/core/lib/Drupal/Core/Cache/MemoryBackend.php
+++ b/core/lib/Drupal/Core/Cache/MemoryBackend.php
@@ -10,6 +10,11 @@
  * Should be used for unit tests and specialist use-cases only, does not
  * store cached items between requests.
  *
+ * The functions ::prepareItem()/::set() use unserialize()/serialize().
+ * It behaves as an external cache backend to avoid changing the cached data
+ * by reference. During prepareItem, the object is not modified because we make
+ * a clone of it.
+ *
  * @ingroup cache
  */
 class MemoryBackend implements CacheBackendInterface, CacheTagsInvalidatorInterface {