extend vs. include in spec form

modules.rb

module Motion
  def default_movement
    "walking"
  end
end

module Flight
  extend Motion

  def movement
    "flying"
  end
end

module Swimming
  include Motion

  def movement
    "swimming"
  end
end

module Speech
  def greet
    "Hello World"
  end
end

class Person
  extend  Motion
  include Speech
end

class Bird
  include Flight
end

class Fish
  include Swimming
end

class Cat; end

modules_spec.rb

require "rspec"
require_relative "modules"

RSpec.describe "Ruby composition methods" do
  describe "a class" do
    context "when it extends a module" do
      it "adds the module methods to the class's class methods" do
        expect(Person.default_movement).to eq("walking")
      end

      it "does not add the module methods to the class's instance methods" do
        person = Person.new

        expect { person.default_movement }.to raise_error(NoMethodError)
      end
    end

    context "when it includes a module" do
      it "add the module methods to the class's instance methods" do
        person = Person.new

        expect(person.greet).to eq("Hello World")
      end
    end
  end

  describe "a module" do
    context "when it extends another module" do
      it "adds the other module's methods to the original module's class methods" do
        bird = Bird.new

        expect(bird.movement).to eq("flying")
        expect { bird.default_movement }.to raise_error(NoMethodError)
      end
    end

    context "when it includes another module" do
      it "adds the other module's methods to the original module's instance methods" do
        fish = Fish.new

        expect(fish.movement).to eq("swimming")
        expect(fish.default_movement).to eq("walking")
      end
    end
  end

  describe "an object" do
    context "when it extends a module" do
      it "gains the modules methods" do
        cat = Cat.new

        cat.extend(Speech)
        expect(cat.greet).to eq("Hello World")
      end
    end

    it "cannot include modules" do
      cat = Cat.new

      expect { cat.include }.to raise_error(NoMethodError)
    end
  end
end