nya.rb

module Helloable
  def hello
    "hello"
  end
end

class Cat
  def hello
    "nya"
  end
end

class Dog
  def hello
    "wan"
  end
end

class Fish
  include Helloable
end

class Human
  include Helloable
end

class Cage
  def initialize(animal)
    @animal = animal
  end
  def hello
    if @animal.respond_to?(:hello)
      p @animal.hello
    else
      p "..."
    end
  end
end

cage = Cage.new(Fish.new)
cage.hello