Ruby/Class/interface

class Duck
  def quack
    "Quack!"
  end
  def swim
    "Paddle paddle paddle..."
  end
end
class Goose
  def honk
    "Honk!"
  end
  def swim
    "Splash splash splash..."
  end
end
class DuckRecording
  def quack
    play
  end
  def play
    "Quack!"
  end
end
def make_it_quack(duck)
  duck.quack
end
make_it_quack(Duck.new)                   # => "Quack!"
make_it_quack(DuckRecording.new)
# TypeException: object not of type Duck