diff --git a/astroid/__init__.py b/astroid/__init__.py
index a16a281512..b2f2c817a7 100644
--- a/astroid/__init__.py
+++ b/astroid/__init__.py
@@ -22,8 +22,11 @@
 pylint... Well, actually the development of this library is essentially
 governed by pylint's needs.
 
-It extends class defined in the python's _ast module with some
-additional methods and attributes. Instance attributes are added by a
+It mimics the class defined in the python's _ast module with some
+additional methods and attributes. New nodes instances are not fully
+compatible with python's _ast.
+
+Instance attributes are added by a
 builder object, which can either generate extended ast (let's call
 them astroid ;) by visiting an existent ast tree or by inspecting living
 object. Methods are added by monkey patching ast classes.