diff --git a/assets/css/site.css b/assets/css/site.css
index 18ac72d..fe19c8f 100644
--- a/assets/css/site.css
+++ b/assets/css/site.css
@@ -28,6 +28,11 @@ a:hover {
     text-decoration: underline;
     color: $primary-color-dark;
 }
+hr {
+    border: 0;
+    height: 1px;
+    background-color: #EEE;
+}
 p {
     max-width: 40em;
     line-height: 1.5;
diff --git a/assets/templates/_header.phtml b/assets/templates/_header.phtml
index ecf8755..9b8a2c8 100644
--- a/assets/templates/_header.phtml
+++ b/assets/templates/_header.phtml
@@ -5,7 +5,8 @@
     <meta charset="utf-8">
 
     <title><?= $title ?? 'PHP Apprentice' ?></title>
-    <meta name="description" content="<?= $description ?? 'A site for learning how to use PHP'?>">
+    <meta name="description" content="<?= $subtitle ?? 'A site for learning how to use PHP' ?>">
+    <meta name="viewport" content="width=device-width, initial-scale=1">
 
     <link rel="stylesheet" href="/css/site.css">
     <link rel="icon" href="/favicon-32.png">
diff --git a/assets/templates/_table_of_contents.phtml b/assets/templates/_table_of_contents.phtml
index d6b368f..c6d9ad9 100644
--- a/assets/templates/_table_of_contents.phtml
+++ b/assets/templates/_table_of_contents.phtml
@@ -11,8 +11,10 @@
         <li><a href="<?= page_path('loops') ?>">Loops</a></li>
         <li><a href="<?= page_path('arrays') ?>">Arrays</a></li>
         <li><a href="<?= page_path('functions') ?>">Functions</a></li>
-        <li><a href="<?= page_path('classes') ?>">Classes</a></li>
-        <li><a href="<?= page_path('classes-visibility') ?>">Classes - Visibility</a></li>
+        <li><a href="<?= page_path('classes') ?>">Classes: Introduction</a></li>
+        <li><a href="<?= page_path('classes-inheritance') ?>">Classes: Inheritance</a></li>
+        <li><a href="<?= page_path('classes-visibility') ?>">Classes: Visibility</a></li>
+        <li><a href="<?= page_path('classes-constructor') ?>">Classes: Constructor</a></li>
     </ol>
     <a href="<?= page_path('credits') ?>">Credits</a>
 </div>
diff --git a/assets/templates/credits.phtml b/assets/templates/credits.phtml
index 71e3bf9..730715a 100644
--- a/assets/templates/credits.phtml
+++ b/assets/templates/credits.phtml
@@ -10,7 +10,7 @@
                 experience for the PHP programming language.
             </p>
             <p>
-                Source Links:
+                Sites used as a reference while writing PHP Apprentice:
                 <ul>
                     <li><a href="https://secure.php.net/" target="_blank">php.net</a></li>
                     <li><a href="https://www.phptherightway.com/" target="_blank">PHP The Right Way</a></li>
@@ -27,6 +27,10 @@
                     <li><a href="https://symfony.com/doc/current/frontend.html" target="_blank">Symfony Encore</a></li>
                 </ul>
             </p>
+            <hr />
+            <p>
+                phpapprentice.com was created and is managed by <a href="https://twitter.com/restoreddev">Andrew Davis</a>.
+            </p>
         </div>
         <div>
             <?php partial('table_of_contents') ?>
diff --git a/assets/templates/default.phtml b/assets/templates/default.phtml
index b91a3d4..c6fcab0 100644
--- a/assets/templates/default.phtml
+++ b/assets/templates/default.phtml
@@ -1,4 +1,4 @@
-<?php partial('header', ['title' => $title, 'description' => $description]) ?>
+<?php partial('header', ['title' => $title, 'subtitle' => $subtitle]) ?>
 
 <div class="menu">
     <button class="menu-button" title="Open Menu">
diff --git a/code/classes-constructor.php b/code/classes-constructor.php
new file mode 100644
index 0000000..34adcc5
--- /dev/null
+++ b/code/classes-constructor.php
@@ -0,0 +1,47 @@
+<?php
+
+// Whenever you create an object in PHP, you put parentheses after the class name.
+// In the previous examples, we always left the parentheses empty.
+class Hat {
+    public $color;
+
+    public function setColor($color)
+    {
+        $this->color = $color;
+    }
+}
+
+$hat = new Hat();
+
+// However, you can actually pass data into the parentheses like a function.
+// The data will be passed to a special function on the class called a constructor.
+class Ballcap
+{
+    public $color;
+
+    public function __construct($color )
+    {
+        $this->color = $color;
+    }
+}
+
+// A constructor is not required, but can make creating a new object easier.
+// They are usually used to define the initial value of a property.
+// Instead of writing:
+$hat = new Hat();
+$hat->setColor('Red');
+
+// You can write:
+$ballcap = new Ballcap('Blue');
+
+// Constructors do not return values because the return value is a always a new object.
+class Tophat
+{
+    public function __construct($color)
+    {
+        return $color;
+    }
+}
+
+// "$tophat" now holds an instance of Tophat, not the color "Grey".
+$tophat = new Tophat('Grey');
diff --git a/code/classes-inheritance.php b/code/classes-inheritance.php
new file mode 100644
index 0000000..f1eed47
--- /dev/null
+++ b/code/classes-inheritance.php
@@ -0,0 +1,62 @@
+<?php
+
+// In PHP, a class can extend another class, inheriting the parent class'
+// properties and methods. To make a class a child of another, use the "extends"
+// keyword after the class name.
+class Vehicle
+{
+    public function drive()
+    {
+        echo "driving...\n";
+    }
+}
+
+class Truck extends Vehicle {}
+
+// This does not error because "Truck" extends "Vehicle".
+$truck = new Truck();
+$truck->drive();
+
+// Even though the child class inherits a parent class' properties and methods,
+// the child can still override the parent.
+class Tractor extends Vehicle
+{
+    public function drive()
+    {
+        echo "driving slowly...\n";
+    }
+}
+
+// Outputs "driving slowly..." instead of "driving...".
+$tractor = new Tractor();
+$tractor->drive();
+
+// A class can use a parent's property or method from the "$this" variable.
+class Motorcycle extends Vehicle
+{
+    public function pushPedal()
+    {
+        $this->drive();
+    }
+}
+
+// Outputs "driving...".
+$cycle = new Motorcycle();
+$cycle->pushPedal();
+
+// If you override a parent's property or method, the "$this" variable will refer to the child's
+// implementation of the property or method. To call the parent's property or method explicity,
+// use the "parent" keyword.
+class Racecar extends Vehicle
+{
+    public function drive()
+    {
+        parent::drive();
+
+        echo "driving even faster...\n";
+    }
+}
+
+// Outputs "driving..." and "driving even faster...".
+$racecar = new Racecar();
+$racecar->drive();
diff --git a/code/classes-visibility.php b/code/classes-visibility.php
index 24e1d90..2833608 100644
--- a/code/classes-visibility.php
+++ b/code/classes-visibility.php
@@ -33,11 +33,28 @@ class Phone2
     }
 }
 
-// The "protected" and "private" keywords work a little differently, but we
-// will learn more about "protected" when we discuss inheritance.
-// However, they both prevent functions and properties from being accessed outside an object.
+// The "protected" and "private" keywords work a little differently.
+// They both prevent functions and properties from being accessed outside an object.
+// However, a method or property marked "protected" can still be accessed by a child object.
 class Phone3
 {
     private $number;
+
     protected $caller;
+
+    public function setNumber($number)
+    {
+        $this->number = $number;
+    }
+}
+
+// In class "Smartphone", the "caller" property is accessible because the parent class
+// has it marked as "protected". However, "Smartphone" cannot access the the "number" property
+// because it is still listed as private.
+class Smartphone extends Phone3
+{
+    public function setCaller($caller)
+    {
+        $this->caller = $caller;
+    }
 }
diff --git a/config.php b/config.php
index f3742b9..e584669 100644
--- a/config.php
+++ b/config.php
@@ -15,77 +15,78 @@ return [
         Page::create('basics', null, 'basics.php', [
             'title' => 'Basics',
             'subtitle' => 'Getting started',
-            'description' => 'Let us get started with the basics.',
             'next' => 'variables',
         ]),
         Page::create('variables', null, 'variables.php', [
             'title' => 'Variables',
             'subtitle' => 'The building blocks of PHP',
-            'description' => '',
             'previous' => 'basics',
             'next' => 'arithmetic',
         ]),
         Page::create('arithmetic', null, 'arithmetic.php', [
             'title' => 'Arithmetic',
             'subtitle' => 'Doing math like a pro',
-            'description' => '',
             'previous' => 'variables',
             'next' => 'comparisons',
         ]),
         Page::create('comparisons', null, 'comparisons.php', [
             'title' => 'Comparisons',
             'subtitle' => 'Equality checking',
-            'description' => '',
             'previous' => 'arithmetic',
             'next' => 'boolean-logic',
         ]),
         Page::create('boolean-logic', null, 'boolean-logic.php', [
             'title' => 'Boolean Logic',
             'subtitle' => 'Is it a yes or a no?',
-            'description' => '',
             'previous' => 'comparisons',
             'next' => 'conditionals',
         ]),
         Page::create('conditionals', null, 'conditionals.php', [
             'title' => 'Conditionals',
             'subtitle' => 'Checking the if before the what',
-            'description' => '',
             'previous' => 'boolean-logic',
             'next' => 'loops',
         ]),
         Page::create('loops', null, 'loops.php', [
             'title' => 'Loops',
             'subtitle' => 'Increase your repetitions',
-            'description' => '',
             'previous' => 'conditionals',
             'next' => 'arrays',
         ]),
         Page::create('arrays', null, 'arrays.php', [
             'title' => 'Arrays',
             'subtitle' => 'Time to make a list',
-            'description' => '',
             'previous' => 'loops',
             'next' => 'functions',
         ]),
         Page::create('functions', null, 'functions.php', [
             'title' => 'Functions',
             'subtitle' => 'Reusable code',
-            'description' => '',
             'previous' => 'arrays',
             'next' => 'classes',
         ]),
         Page::create('classes', null, 'classes.php', [
             'title' => 'Classes',
             'subtitle' => 'Object-oriented programming',
-            'description' => '',
             'previous' => 'functions',
+            'next' => 'classes-inheritance',
+        ]),
+        Page::create('classes-inheritance', null, 'classes-inheritance.php', [
+            'title' => 'Classes: Inheritance',
+            'subtitle' => 'Extend your objects',
+            'previous' => 'classes',
             'next' => 'classes-visibility',
         ]),
         Page::create('classes-visibility', null, 'classes-visibility.php', [
-            'title' => 'Classes Visibility',
+            'title' => 'Classes: Visibility',
             'subtitle' => 'Privatizing your objects',
-            'description' => '',
-            'previous' => 'classes',
+            'previous' => 'classes-inheritance',
+            'next' => 'classes-constructor',
+        ]),
+        Page::create('classes-constructor', null, 'classes-constructor.php', [
+            'title' => 'Classes: Constructor',
+            'subtitle' => 'Construct your objects',
+            'previous' => 'classes-visibility',
             'next' => '',
         ]),
     ],
diff --git a/src/Build.php b/src/Build.php
index 2dda936..e239914 100644
--- a/src/Build.php
+++ b/src/Build.php
@@ -96,6 +96,10 @@ class Build
     private function buildPage(Page $page): string
     {
         if (!empty($page->code)) {
+            if (!file_exists(config('code_dir') . '/' . $page->code)) {
+                throw new \Exception('Code file not found: ' . $page->code);
+            }
+
             $code = file_get_contents(config('code_dir') . '/' . $page->code);
             $page->variables['code'] = $code;
         }