Add undef_method And The Inheritance Hierarchy as a ruby til.

README.md

@@ -117,6 +117,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Question Mark Operator](ruby/question-mark-operator.md)
 - [Squeeze Out The Extra Space](ruby/squeeze-out-the-extra-space.md)
 - [Summing Collections](ruby/summing-collections.md)
+- [`undef_method` And The Inheritance Hierarchy](ruby/undef-method-and-the-inheritance-hierarchy.md)
 
 ### tmux
 

ruby/undef-method-and-the-inheritance-hierarchy.md

@@ -0,0 +1,52 @@
+# `undef_method` And The Inheritance Hierarchy
+
+As the docs state, Ruby's
+[`undef_method`](http://ruby-doc.org/core-2.2.0/Module.html#method-i-undef_method)
+
+> prevents the current class from responding to calls to the named method.
+
+This means you can do some weird things to the inheritance hierarchy. I'll
+use the following code example to illustrate.
+
+```ruby
+class Phone
+  def ring
+    puts 'brrrrriiing'
+  end
+end
+
+class Smartphone < Phone
+  def ring
+    puts 'boop beep boop'
+  end
+end
+
+class Iphone < Smartphone
+end
+
+smartphone = Smartphone.new
+iphone = Iphone.new
+
+smartphone.ring
+#=> boop beep boop
+iphone.ring
+#=> boop beep boop
+```
+
+Everything works as expect. Now, I'll use `undef_method`.
+
+```ruby
+class Smartphone
+  undef_method(:ring)
+end
+
+smartphone.ring
+#=> NoMethodError: undefined method `ring' for #<Smartphone:0x007fd0a20b7960>
+iphone.ring
+#=> NoMethodError: undefined method `ring' for #<Iphone:0x007fd0a20b7938>
+```
+
+Not only have instances of `Smartphone` been prevented from responding to
+`ring`, but any subclasses of `Smartphone` that call `ring` will get tripped
+up when traversing the inheritance hierarchy in search of a definition of
+`ring`.