diff --git a/exercises/exercise-properties/problem.hh b/exercises/exercise-properties/problem.hh
index bc0cf9451316125f4f1bb0726d9edf3bb37fcdc2..b3391869c434efaf53dd816aa76538ac97276136 100644
--- a/exercises/exercise-properties/problem.hh
+++ b/exercises/exercise-properties/problem.hh
@@ -176,6 +176,7 @@ public:
      */
     NumEqVector neumannAtPos(const GlobalPosition &globalPos) const
     {
+// TODO: reformulate the neumann boundary condition's values in terms of volume instead of mass injected per meter and second
         NumEqVector values(0.0);
         if (onInlet_(globalPos))
             values[contiDNAPLEqIdx] = -0.04; // kg / (m * s)