diff --git a/core/lib/Drupal/Core/Cache/MemoryBackend.php b/core/lib/Drupal/Core/Cache/MemoryBackend.php
index 76bff41..26454cf 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. In ::prepareItem(), the object is not modified by the call of to
+ * unserialize() because we make a clone of it.
+ *
  * @ingroup cache
  */
 class MemoryBackend implements CacheBackendInterface, CacheTagsInvalidatorInterface {