diff --git a/README.md b/README.md
index 4d11ed9..f50efe7 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1699 TILs and counting..._
+_1700 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -1357,6 +1357,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Create a CSV::Table Object](ruby/create-a-csv-table-object.md)
 - [Create A Hash From An Array Of Arrays](ruby/create-a-hash-from-an-array-of-arrays.md)
 - [Create Listing Of All Middleman Pages](ruby/create-listing-of-all-middleman-pages.md)
+- [Create Mock Class That Can Be Overridden](ruby/create-mock-class-that-can-be-overridden.md)
 - [Create Named Structs With Struct.new](ruby/create-named-structs-with-struct-new.md)
 - [Create Thumbnail Image For A PDF](ruby/create-thumbnail-image-for-a-pdf.md)
 - [Decompose Unicode Character With Diacritic Mark](ruby/decompose-unicode-character-with-diacritic-mark.md)
diff --git a/ruby/create-mock-class-that-can-be-overridden.md b/ruby/create-mock-class-that-can-be-overridden.md
new file mode 100644
index 0000000..9852cc8
--- /dev/null
+++ b/ruby/create-mock-class-that-can-be-overridden.md
@@ -0,0 +1,85 @@
+# Create Mock Class That Can Be Overridden
+
+Let's say I've defined a `MockTwilioClient` class for my system tests so that
+they don't have to actually make calls out to the Twilio API.
+
+```ruby
+class MockTwilioClient
+  def send_sms_message
+    "MSG_SID_123"
+  end
+end
+```
+
+Now, any test that is exercising behavior that uses those parts of the
+`TwilioClient` can mock those calls in a predictable way.
+
+The above class always is successful when `send_sms_message` is called. What if
+we want to simulate an error response? We need a way to override the client for
+specific testing scenarios.
+
+Let's create a helper that can create `MockTwilioClient` instances, either as is
+or with overrides.
+
+```ruby
+def create_mock_twilio_client(&block)
+  if block_given?
+    Class.new(MockTwilioClient, &block).new
+  else
+    MockTwilioClient.new
+  end
+end
+```
+
+If we pass it a block with specific method overrides, it will create a one-off
+anonymous subclass of `MockTwilioClient` with that block which effectively
+overrides the parent class's methods.
+
+We can put this to use like so:
+
+```ruby
+require 'test_helper'
+
+class SomeSystemTest < SystemTestCase
+  class MockTwilioClient
+    def send_sms_message
+      "MSG_SID_123"
+    end
+  end
+
+  def create_mock_twilio_client(&block)
+    if block_given?
+      Class.new(MockTwilioClient, &block).new
+    else
+      MockTwilioClient.new
+    end
+  end
+
+  test "send message to customer" do
+    mock_client = create_mock_twilio_client
+
+    TwilioClient.stub(:new, mock_client) do
+      # some action that uses `send_sms_message`
+      
+      # some assertions ...
+    end
+  end
+
+  test "fail to send message to customer" do
+    mock_client = create_mock_twilio_client do
+      def send_sms_message
+        raise "Failed to send message"
+      end
+    end
+
+    TwilioClient.stub(:new, mock_client) do
+      # some action that uses `send_sms_message`
+      
+      # some assertions ...
+    end
+  end
+end
+```
+
+In the second test case, I override the _success path_ with a version of
+`send_sms_message` that raises an error.