diff --git a/Basic/Solid/ShapeInterface.php b/Basic/Solid/ShapeInterface.php
index aae0f75..063ac22 100644
--- a/Basic/Solid/ShapeInterface.php
+++ b/Basic/Solid/ShapeInterface.php
@@ -2,4 +2,5 @@
 
 interface ShapeInterface {
   public function area();
+  public function volume();
 }
\ No newline at end of file
diff --git a/Basic/Solid/doc.md b/Basic/Solid/doc.md
index 002bd2e..9f70d7d 100644
--- a/Basic/Solid/doc.md
+++ b/Basic/Solid/doc.md
@@ -10,4 +10,8 @@ Let q(x) be a property provable about objects of x of type T. Then q(y) should b
 q(x) x of type T 
 q(y) y of type S (subtype of T)
 
-This means that every subclass or derived class should be substitutable for their base or parent class.
\ No newline at end of file
+This means that every subclass or derived class should be substitutable for their base or parent class.
+
+# Interface Segregation Principle
+A client should never be forced to implement an interface that it doesn’t use, or clients shouldn’t be forced to depend on methods they do not use.
+