diff --git a/chap03.md b/chap03.md
index d36657d..169182b 100644
--- a/chap03.md
+++ b/chap03.md
@@ -407,6 +407,24 @@ House mansion = new House { Name = "Mansion",
 
 ### 3.2.1 Polymorphism
 
+**Polymorphism (多态)**: References (e.g. class) are *polymorphic* (i.e. a variable of type `x` can refer to an object that subclasses `x`)
+* As all subclasses have field/properties/methods of superclass, so it's safe
+
+```csharp
+// Define a method that try to access superclass
+public static void Display (Asset asset)
+{
+    System.Console.WriteLine(asset.Name)
+}
+
+// As Stock is subclass of Asset, Display can access it
+Stock msft = new Stock ...;
+House mansion = new House ...;
+
+Display(msft); // msft is subclass of asset
+Display(mansion); // mansion is subclass of asset
+```
+
 ### 3.2.2 Casting and Reference Conversions
 
 ### 3.2.3 Virtual Function Members