diff --git a/elastica/timestepper/explicit_steppers.py b/elastica/timestepper/explicit_steppers.py
index 780e9049..1d686b9a 100644
--- a/elastica/timestepper/explicit_steppers.py
+++ b/elastica/timestepper/explicit_steppers.py
@@ -116,6 +116,10 @@ def stage(
         """
         In-place update of system states with Runge-Kutta 4 integration
         """
+
+        # TODO: Try to avoid deepcopying the whole system collection
+        # Related to #33: save/restart module
+        # Maybe search for low-storage RK scheme
         k1_system = SystemCollection  # Alias
         k2_system = deepcopy(SystemCollection)
         k3_system = deepcopy(SystemCollection)