diff --git a/pina/domain/operation_interface.py b/pina/domain/operation_interface.py
index 0300f52..e42d37e 100644
--- a/pina/domain/operation_interface.py
+++ b/pina/domain/operation_interface.py
@@ -43,7 +43,10 @@ class OperationInterface(DomainInterface, metaclass=ABCMeta):
         :return: All the variables defined in ``__init__`` in order.
         :rtype: list[str]
         """
-        return self.geometries[0].variables
+        variables = []
+        for geom in self.geometries:
+            variables += geom.variables
+        return sorted(list(set(variables)))
 
     @abstractmethod
     def is_inside(self, point, check_border=False):
diff --git a/pina/domain/union_domain.py b/pina/domain/union_domain.py
index 0af8e1b..91aa5fb 100644
--- a/pina/domain/union_domain.py
+++ b/pina/domain/union_domain.py
@@ -39,13 +39,6 @@ class Union(OperationInterface):
             set([geom.sample_modes for geom in self.geometries])
         )
 
-    @property
-    def variables(self):
-        variables = []
-        for geom in self.geometries:
-            variables += geom.variables
-        return list(set(variables))
-
     def is_inside(self, point, check_border=False):
         """
         Check if a point is inside the ``Union`` domain.