Testing the Invisibility superpower by creating a sort of dumb target class 'Person' to give the power to in the test.

invisibility_spec.rb

module Invisibility
  def activate
    @visible = false
  end

  def deactivate
    @visible = true
  end
end

class Person
  attr_reader :visible
  def initialize
    @visible = true
  end

  def visible?
    @visible
  end
end

describe "invisibility" do

  it "can be activated" do
    Person.send :include, Invisibility
    bob = Person.new
    bob.should respond_to :activate
    bob.activate
    bob.should_not be_visible
  end

  it "can be deactivated" do
    Person.send :include, Invisibility
    bob = Person.new
    bob.should respond_to :deactivate
    bob.deactivate
    bob.should be_visible
  end

end