gistfile1.builder

class Animal < Struct.new(:species)
end

o1 = Animal.new(:duck)
o2 = Animal.new(:duck)
o3 = Animal.new(:goose)

[o1,o2,o3].each do |animal|
  sound = animal.species == :duck ? 'Quack' : 'Say What?'
  animal.instance_variable_set :@sound, sound
  def animal.speak ; instance_variable_get(:@sound) ; end
end

o1.speak # => "Quack"
o2.speak # => "Quack"
o3.speak # => "Say What?"