diff --git a/.gitignore b/.gitignore
index e43b0f9..5b2294c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,3 @@
 .DS_Store
+_build
+
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..d74134c
--- /dev/null
+++ b/README.md
@@ -0,0 +1,50 @@
+Sector Five II
+==============
+
+Extended edition of the game, based on a Tutorial from Learn Game Programming
+with Ruby by Mark Sobkowicz.
+
+You can read the [credits](credits.txt) seperately.
+
+Installation
+============
+
+The are two ways to run the game. One is setting up Ruby in your environment,
+along all the necessary libraries. Download the source code and run the game.
+
+The other is to go to this repository's releases page, and download a bundled
+version. For now, I only provide a macOS bundle, but I can soon provide a
+Windows one as well.
+
+Running or developing on macOS
+==============================
+
+1. Install the latest Ruby version
+2. brew install sdl2 libogg libvorbis​
+3. gem install gosu
+4. gem install chipmunk
+
+Running or developing on Windows
+================================
+
+[...Help Needed...]
+Need someone with Windows to contribute this bit. They need to be able to run
+this game on their system.
+
+Credits
+=======
+The starter base code comes from a tutorial from Learn Game Programming
+with Ruby by [Mark Sobkowicz](https://twitter.com/MarkSobkowicz).
+
+You can read the rest of the [credits](credits.txt) seperately. It includes the
+attributions for the sounds and images used in this game.
+
+Contributing
+============
+
+Please follow the [CONTRIBUTING](CONTRIBUTING) guide.
+
+Contributors
+============
+
+- [petros](https://twitter.com/amiridis) ([petros.blog/games](https://petros.blog/games))
diff --git a/bullet.rb b/bullet.rb
index 17a3600..31eaca5 100644
--- a/bullet.rb
+++ b/bullet.rb
@@ -1,7 +1,8 @@
-class Bullet
+require_relative 'sprite'
+
+class Bullet < Sprite
   SPEED = 5
-  attr_reader :x, :y, :radius
-  
+
   def initialize(window, x, y, angle)
     @x = x
     @y = y
@@ -10,16 +11,16 @@ def initialize(window, x, y, angle)
     @radius = 3
     @window = window
   end
-  
+
   def move
     @x += Gosu.offset_x(@direction, SPEED)
     @y += Gosu.offset_y(@direction, SPEED)
   end
-  
+
   def draw
-    @image.draw(@x - @radius, @y - @radius, 1) 
+    @image.draw(@x - @radius, @y - @radius, 1)
   end
-  
+
   def onscreen?
     right = @window.width + @radius
     left = -@radius
diff --git a/credit.rb b/credit.rb
index 5432311..4b808e8 100644
--- a/credit.rb
+++ b/credit.rb
@@ -1,22 +1,22 @@
 class Credit
   SPEED = 1
   attr_reader :y
-  
+
   def initialize(window, text, x, y)
     @x = x
     @y = @initial_y = y
     @text = text
     @font = Gosu::Font.new(24)
   end
-  
+
   def move
     @y -= SPEED
   end
-  
+
   def draw
     @font.draw_text(@text, @x, @y, 1)
   end
-  
+
   def reset
     @y = @initial_y
   end
diff --git a/end_scenes.rb b/end_scenes.rb
new file mode 100644
index 0000000..6316e29
--- /dev/null
+++ b/end_scenes.rb
@@ -0,0 +1,78 @@
+require 'gosu'
+require_relative 'scene'
+require_relative 'credit'
+
+class EndScene < Scene
+  def initialize(enemies_destroyed)
+    @enemies_destroyed = enemies_destroyed
+    @bottom_message = "Press P to play again, or Q to quit."
+    @message_font = Gosu::Font.new(28)
+    @credits = []
+    y = 700
+    File.open('credits.txt').each do |line|
+      @credits.push(Credit.new(self, line.chomp, 100, y))
+      y += 30
+    end
+    @scene = :end
+    @end_music = Gosu::Song.new('sounds/from_here.ogg')
+    @end_music.play(true)
+  end
+
+  def button_down(id)
+    if id == Gosu::KbP
+      Game.current_scene = FirstWaveScene.new
+    elsif id == Gosu::KbQ
+      Game.window.close
+    end
+  end
+
+  def update
+    @credits.each do |credit|
+      credit.move
+    end
+    if @credits.last.y < 150
+      @credits.each do |credit|
+        credit.reset
+      end
+    end
+  end
+
+  def draw
+    Gosu.clip_to(50, 140, 700, 360) do
+      @credits.each do |credit|
+        credit.draw
+      end
+    end
+    Gosu.draw_line(0, 140, Gosu::Color::RED, Game::WINDOW_WIDTH, 140, Gosu::Color::RED)
+    @message_font.draw_text(@message, 40, 40, 1, 1, 1, Gosu::Color::FUCHSIA)
+    @message_font.draw_text(@message2, 40, 75, 1, 1, 1, Gosu::Color::FUCHSIA)
+    Gosu.draw_line(0, 500, Gosu::Color::RED, Game::WINDOW_WIDTH, 500, Gosu::Color::RED)
+    @message_font.draw_text(@bottom_message, 180, 540, 1, 1, 1, Gosu::Color::AQUA)
+  end
+end
+
+class EndCountReachedScene < EndScene
+  def initialize(enemies_destroyed)
+    super(enemies_destroyed)
+    @message = "You made it! You destroyed #{enemies_destroyed} ships"
+    @message2 = "and #{Game::MAX_ENEMIES - enemies_destroyed} reached the base."
+  end
+end
+
+class EndHitByEnemyScene < EndScene
+  def initialize(enemies_destroyed)
+    super(enemies_destroyed)
+    @message = "You were struck by an enemy ship."
+    @message2 = "Before your ship was destroyed, "
+    @message2 += "you took out #{enemies_destroyed} enemy ships."
+  end
+end
+
+class EndOffTopScene < EndScene
+  def initialize(enemies_destroyed)
+    super(enemies_destroyed)
+    @message = "You got too close to the enemy mother ship."
+    @message2 = "Before your ship was destroyed, "
+    @message2 += "you took out #{enemies_destroyed} enemy ships."
+  end
+end
diff --git a/enemy.rb b/enemy.rb
index 84e7351..cb5a6a4 100644
--- a/enemy.rb
+++ b/enemy.rb
@@ -1,6 +1,6 @@
-class Enemy
-  attr_reader :x, :y, :radius
-  
+require_relative 'sprite'
+
+class Enemy < Sprite
   def initialize(window)
     @radius = 20
     @x = rand(window.width - 2 * @radius) + @radius
@@ -8,11 +8,11 @@ def initialize(window)
     @image = Gosu::Image.new('images/enemy.png')
     @speed = rand(1..6)
   end
-  
+
   def move
     @y += @speed
   end
-  
+
   def draw
     @image.draw(@x - @radius, @y - @radius, 1)
   end
diff --git a/explosion.rb b/explosion.rb
index f98b6e8..d0ca225 100644
--- a/explosion.rb
+++ b/explosion.rb
@@ -1,6 +1,8 @@
-class Explosion
-  attr_reader :x, :y, :radius, :finished
-  
+require_relative 'sprite'
+
+class Explosion < Sprite
+  attr_reader :finished
+
   def initialize(window, x, y)
     @x = x
     @y = y
@@ -9,7 +11,7 @@ def initialize(window, x, y)
     @image_index = 0
     @finished = false
   end
-  
+
   def draw
     if @image_index < @images.count
       @images[@image_index].draw(@x - @radius, @y - @radius, 2)
diff --git a/first_wave_scene.rb b/first_wave_scene.rb
new file mode 100644
index 0000000..13ba9c0
--- /dev/null
+++ b/first_wave_scene.rb
@@ -0,0 +1,145 @@
+require 'gosu'
+require_relative 'game'
+require_relative 'scene'
+require_relative 'player'
+require_relative 'enemy'
+require_relative 'bullet'
+require_relative 'explosion'
+require_relative 'end_scenes'
+
+class FirstWaveScene < Scene
+  def initialize
+    @scene = :first_wave
+    @player = Player.new(Game.window)
+    @enemies = []
+    @bullets = []
+    @explosions = []
+    @enemies_appeared = 0
+    @enemies_appeared_font = Gosu::Font.new(18, bold: true)
+    @enemies_destroyed = 0
+    @enemies_destroyed_font = Gosu::Font.new(18, bold: true)
+    @enemies_escaped_font = Gosu::Font.new(18, bold: true)
+    @game_music = Gosu::Song.new('sounds/cephalopod.ogg')
+    @game_music.play(true)
+    @explosion_sound = Gosu::Sample.new('sounds/explosion.ogg')
+    @shooting_sound = Gosu::Sample.new('sounds/shoot.ogg')
+    @teleport_sound = Gosu::Sample.new('sounds/teleport.wav')
+  end
+
+  def button_down(id)
+    if id == Gosu::KbSpace
+      @bullets.push Bullet.new(Game.window, @player.x, @player.y, @player.angle)
+      @shooting_sound.play(0.3)
+    end
+  end
+
+  def update
+    @player.turn_left if Gosu.button_down?(Gosu::KbLeft)
+    @player.turn_right if Gosu.button_down?(Gosu::KbRight)
+    @player.accelerate if Gosu.button_down?(Gosu::KbUp)
+    @player.move
+    if rand < Game::ENEMY_FREQUENCY
+      @enemies.push Enemy.new(Game.window)
+      @enemies_appeared += 1
+    end
+    @enemies.each do |enemy|
+      enemy.move
+    end
+    @bullets.each do |bullet|
+      bullet.move
+    end
+    # Detect if a bullet hits an enemy, and push an explosion.
+    @enemies.dup.each do |enemy|
+      @bullets.dup.each do |bullet|
+        if bullet.collides_with?(enemy)
+          @enemies.delete enemy
+          @enemies_destroyed += 1
+          @bullets.delete bullet
+          @explosions.push Explosion.new(Game.window, enemy.x, enemy.y)
+          @explosion_sound.play
+        end
+      end
+    end
+    # Chain explosions. If an enemy is near an ongoing explosion, it explodes
+    # too.
+    @enemies.dup.each do |enemy|
+      @explosions.dup.each do |explosion|
+        if enemy.collides_with?(explosion)
+          @enemies.delete enemy
+          @enemies_destroyed += 1
+          @explosions.push Explosion.new(Game.window, enemy.x, enemy.y)
+          @explosion_sound.play
+        end
+      end
+    end
+    # Remove explosions that have finished animating.
+    @explosions.dup.each do |explosion|
+      @explosions.delete explosion if explosion.finished
+    end
+    # Remove enemies that have moved beyond the bottom of the screen.
+    @enemies.dup.each do |enemy|
+      if enemy.y > Game::WINDOW_HEIGHT + enemy.radius
+        @enemies.delete enemy
+        @teleport_sound.play(0.3)
+      end
+    end
+    # Remove bullets that have moved out of the screen boundaries.
+    @bullets.dup.each do |bullet|
+      @bullets.delete bullet unless bullet.onscreen?
+    end
+    if @enemies_appeared > Game::MAX_ENEMIES
+      Game.current_scene = EndCountReachedScene.new(@enemies_destroyed)
+    end
+    @enemies.each do |enemy|
+      #distance = Gosu.distance(enemy.x, enemy.y, @player.x, @player.y)
+      #if distance < @player.radius + enemy.radius
+      if enemy.collides_with?(@player)
+        Game.current_scene = EndHitByEnemyScene.new(@enemies_destroyed)
+      end
+    end
+    if @player.y < @player.radius
+      Game.current_scene = EndOffTopScene.new(@enemies_destroyed)
+    end
+  end
+
+  def draw
+    @player.draw
+    @enemies.each do |enemy|
+      enemy.draw
+    end
+    @bullets.each do |bullet|
+      bullet.draw
+    end
+    @explosions.each do |explosion|
+      explosion.draw
+    end
+    @enemies_appeared_font.draw_text(
+      "Fleet: #{Game::MAX_ENEMIES - @enemies_appeared}",
+      460,
+      10,
+      1,
+      1,
+      1,
+      Gosu::Color::WHITE
+    )
+    @enemies_destroyed_font.draw_text(
+      "Destroyed: #{@enemies_destroyed}",
+      540,
+      10,
+      1,
+      1,
+      1,
+      Gosu::Color::GREEN
+    )
+    @enemies_escaped_font.draw_text(
+      "Escaped: #{Game::MAX_ENEMIES - @enemies_destroyed}",
+      660,
+      10,
+      1,
+      1,
+      1,
+      Gosu::Color::RED
+    )
+  end
+
+end
\ No newline at end of file
diff --git a/game.rb b/game.rb
new file mode 100644
index 0000000..85507ff
--- /dev/null
+++ b/game.rb
@@ -0,0 +1,17 @@
+require_relative 'start_scene'
+
+class Game
+  WINDOW_WIDTH = 800
+  WINDOW_HEIGHT = 600
+  ENEMY_FREQUENCY = 0.05
+  MAX_ENEMIES = 100
+
+  class << self
+     attr_accessor :current_scene, :window
+  end
+
+  def initialize(window)
+    Game.current_scene = StartScene.new
+    Game.window = window
+  end
+end
diff --git a/main.rb b/main.rb
index 9dc6494..703615d 100644
--- a/main.rb
+++ b/main.rb
@@ -1,250 +1,27 @@
+################################################################################
+#
+# Sector Five II
+#
 require 'gosu'
-require_relative 'player'
-require_relative 'enemy'
-require_relative 'bullet'
-require_relative 'explosion'
-require_relative 'credit'
+require_relative 'game'
 
 class SectorFive < Gosu::Window
-  WIDTH = 800
-  HEIGHT = 600
-  ENEMY_FREQUENCY = 0.05
-  MAX_ENEMIES = 100
-  
   def initialize
-    super(WIDTH, HEIGHT)
+    super(Game::WINDOW_WIDTH, Game::WINDOW_HEIGHT)
     self.caption = 'Sector Five II'
-    @background_image = Gosu::Image.new('images/start_screen.png')
-    @scene = :start
-    @start_music = Gosu::Song.new('sounds/Lost Frontier.ogg')
-    @start_music.play(true)
+    Game.new(self)
   end
-  
+
   def button_down(id)
-    case @scene
-    when :start
-      button_down_start(id)
-    when :game
-      button_down_game(id)
-    when :end
-      button_down_end(id)
-    end
-  end
-  
-  def button_down_start(id)
-    initialize_game
-  end
-  
-  def button_down_game(id)
-    if id == Gosu::KbSpace
-      @bullets.push Bullet.new(self, @player.x, @player.y, @player.angle)
-      @shooting_sound.play(0.3)
-    end
-  end
-  
-  def button_down_end(id)
-    if id == Gosu::KbP
-      initialize_game
-    elsif id == Gosu::KbQ
-      close
-    end
+    Game.current_scene.button_down(id)
   end
-  
-  def initialize_game
-    @player = Player.new(self)
-    @enemies = []
-    @bullets = []
-    @explosions = []
-    @enemies_appeared = 0
-    @enemies_appeared_font = Gosu::Font.new(18, bold: true)
-    @enemies_destroyed = 0
-    @enemies_destroyed_font = Gosu::Font.new(18, bold: true)
-    @enemies_escaped_font = Gosu::Font.new(18, bold: true)
-    @scene = :game
-    @game_music = Gosu::Song.new('sounds/Cephalopod.ogg')
-    @game_music.play(true)
-    @explosion_sound = Gosu::Sample.new('sounds/explosion.ogg')
-    @shooting_sound = Gosu::Sample.new('sounds/shoot.ogg')
-    @teleport_sound = Gosu::Sample.new('sounds/teleport.wav')
-  end
-  
-  def initialize_end(fate)
-    case fate
-    when :count_reached
-      @message = "You made it! You destroyed #{@enemies_destroyed} ships"
-      @message2 = "and #{100-@enemies_destroyed} reached the base."
-    when :hit_by_enemy
-      @message = "You were struck by an enemy ship."
-      @message2 = "Before your ship was destroyed, "
-      @message2 += "you took out #{@enemies_destroyed} enemy ships."
-    when :off_top
-      @message = "You got too close to the enemy mother ship."
-      @message2 = "Before your ship was destroyed, "
-      @message2 += "you took out #{@enemies_destroyed} enemy ships."
-    end
-    @bottom_message = "Press P to play again, or Q to quit."
-    @message_font = Gosu::Font.new(28)
-    @credits = []
-    y = 700
-    File.open('credits.txt').each do |line|
-      @credits.push(Credit.new(self, line.chomp, 100, y))
-      y += 30
-    end
-    @scene = :end
-    @end_music = Gosu::Song.new('sounds/FromHere.ogg')
-    @end_music.play(true)
-  end
-  
+
   def update
-    case @scene
-    when :game
-      update_game
-    when :end
-      update_end
-    end
+    Game.current_scene.update
   end
-  
-  def update_game
-    @player.turn_left if button_down?(Gosu::KbLeft)
-    @player.turn_right if button_down?(Gosu::KbRight)
-    @player.accelerate if button_down?(Gosu::KbUp)
-    @player.move
-    if rand < ENEMY_FREQUENCY
-      @enemies.push Enemy.new(self)
-      @enemies_appeared += 1
-    end
-    @enemies.each do |enemy|
-      enemy.move
-    end
-    @bullets.each do |bullet|
-      bullet.move
-    end
-    # Detect if a bullet hits an enemy, and push an explosion.
-    @enemies.dup.each do |enemy|
-      @bullets.dup.each do |bullet|
-        distance = Gosu.distance(enemy.x, enemy.y, bullet.x, bullet.y)
-        if distance < enemy.radius + bullet.radius
-          @enemies.delete enemy
-          @enemies_destroyed += 1
-          @bullets.delete bullet
-          @explosions.push Explosion.new(self, enemy.x, enemy.y)
-          @explosion_sound.play
-        end
-      end
-    end
-    # Chain explosions. If an enemy is near an ongoing explosion, it explodes
-    # too.
-    @enemies.dup.each do |enemy|
-      @explosions.dup.each do |explosion|
-        distance = Gosu.distance(enemy.x, enemy.y, explosion.x, explosion.y)
-        if distance < enemy.radius + explosion.radius
-          @enemies.delete enemy
-          @enemies_destroyed += 1
-          @explosions.push Explosion.new(self, enemy.x, enemy.y)
-          @explosion_sound.play
-        end
-      end
-    end
-    # Remove explosions that have finished animating.
-    @explosions.dup.each do |explosion|
-      @explosions.delete explosion if explosion.finished
-    end
-    # Remove enemies that have moved beyond the bottom of the screen.
-    @enemies.dup.each do |enemy|
-      if enemy.y > HEIGHT + enemy.radius
-        @enemies.delete enemy
-        @teleport_sound.play(0.3)
-      end
-    end
-    # Remove bullets that have moved out of the screen boundaries.
-    @bullets.dup.each do |bullet|
-      @bullets.delete bullet unless bullet.onscreen?
-    end
-    initialize_end(:count_reached) if @enemies_appeared > MAX_ENEMIES
-    @enemies.each do |enemy|
-      distance = Gosu.distance(enemy.x, enemy.y, @player.x, @player.y)
-      initialize_end(:hit_by_enemy) if distance < @player.radius + enemy.radius
-    end
-    initialize_end(:off_top) if @player.y < @player.radius
-  end
-  
-  def update_end
-    @credits.each do |credit|
-      credit.move
-    end
-    if @credits.last.y < 150
-      @credits.each do |credit|
-        credit.reset
-      end
-    end
-  end
-  
+
   def draw
-    case @scene
-    when :start
-      draw_start
-    when :game
-      draw_game
-    when :end
-      draw_end
-    end
-  end
-  
-  def draw_start
-    @background_image.draw(0,0,0)
-  end
-  
-  def draw_game
-    @player.draw
-    @enemies.each do |enemy|
-      enemy.draw
-    end
-    @bullets.each do |bullet|
-      bullet.draw
-    end
-    @explosions.each do |explosion|
-      explosion.draw
-    end
-    @enemies_appeared_font.draw_text(
-      "Fleet: #{MAX_ENEMIES - @enemies_appeared}",
-      460,
-      10,
-      1,
-      1,
-      1,
-      Gosu::Color::WHITE
-    )
-    @enemies_destroyed_font.draw_text(
-      "Destroyed: #{@enemies_destroyed}",
-      560,
-      10,
-      1,
-      1,
-      1,
-      Gosu::Color::GREEN
-    )
-    @enemies_escaped_font.draw_text(
-      "Escaped: #{MAX_ENEMIES - @enemies_destroyed}",
-      660,
-      10,
-      1,
-      1,
-      1,
-      Gosu::Color::RED
-    )
-  end
-  
-  def draw_end
-    clip_to(50, 140, 700, 360) do
-      @credits.each do |credit|
-        credit.draw
-      end
-    end
-    draw_line(0, 140, Gosu::Color::RED, WIDTH, 140, Gosu::Color::RED)
-    @message_font.draw_text(@message, 40, 40, 1, 1, 1, Gosu::Color::FUCHSIA)
-    @message_font.draw_text(@message2, 40, 75, 1, 1, 1, Gosu::Color::FUCHSIA)
-    draw_line(0, 500, Gosu::Color::RED, WIDTH, 500, Gosu::Color::RED)
-    @message_font.draw_text(@bottom_message, 180, 540, 1, 1, 1, Gosu::Color::AQUA)
+    Game.current_scene.draw
   end
 end
 
diff --git a/player.rb b/player.rb
index c26bc34..6233663 100644
--- a/player.rb
+++ b/player.rb
@@ -1,10 +1,12 @@
-class Player
+require_relative 'sprite'
+
+class Player < Sprite
   ROTATION_SPEED = 3
   ACCELERATION = 2
   FRICTION = 0.9
-  
-  attr_reader :x, :y, :angle, :radius
-  
+
+  attr_reader :angle
+
   def initialize(window)
     @x = 200
     @y = 200
@@ -15,24 +17,24 @@ def initialize(window)
     @radius = 20
     @window = window
   end
-  
+
   def draw
     @image.draw_rot(@x, @y, 1, @angle)
   end
-  
+
   def turn_right
     @angle += ROTATION_SPEED
   end
-  
+
   def turn_left
     @angle -= ROTATION_SPEED
   end
-  
+
   def accelerate
     @velocity_x += Gosu.offset_x(@angle, ACCELERATION)
     @velocity_y += Gosu.offset_y(@angle, ACCELERATION)
   end
-  
+
   def move
     @x += @velocity_x
     @y += @velocity_y
diff --git a/scene.rb b/scene.rb
new file mode 100644
index 0000000..97f7159
--- /dev/null
+++ b/scene.rb
@@ -0,0 +1,13 @@
+class Scene
+  def initialize
+  end
+
+  def button_down(id)
+  end
+
+  def update
+  end
+
+  def draw
+  end
+end
diff --git a/sprite.rb b/sprite.rb
new file mode 100644
index 0000000..9fdc22c
--- /dev/null
+++ b/sprite.rb
@@ -0,0 +1,14 @@
+require 'gosu'
+
+class Sprite
+  attr_reader :x, :y, :radius
+
+  def initialize
+  end
+
+  def collides_with?(sprite)
+    distance = Gosu.distance(x, y, sprite.x, sprite.y)
+    return distance < @radius + sprite.radius
+  end
+
+end
diff --git a/start_scene.rb b/start_scene.rb
new file mode 100644
index 0000000..4376e54
--- /dev/null
+++ b/start_scene.rb
@@ -0,0 +1,23 @@
+require 'gosu'
+require_relative 'scene'
+require_relative 'game'
+require_relative 'first_wave_scene'
+
+class StartScene < Scene
+  def initialize
+    @background_image = Gosu::Image.new('images/start_screen.png')
+    @start_music = Gosu::Song.new('sounds/lost_frontier.ogg')
+    @start_music.play(true)
+  end
+  
+  def button_down(id)
+    Game.current_scene = FirstWaveScene.new
+    super
+  end
+  
+  def draw
+    @background_image.draw(0,0,0)
+    super
+  end
+  
+end
\ No newline at end of file