extend.rb

class Foo
  attr_reader :opt
  def initialize(opt)
    @opt = opt
  end
  
  def meth_in_bar
    :from_foo
  end
end

module Bar
  def meth_in_bar
    :from_bar
  end
end

a, b, c = Foo.new("Module into object via extend"), Foo.new("Module into object via eigenclass"), Foo.new("Vanilla foo")

a.extend(Bar)
class << b
  include Bar
end

[a, b,c].map{|e| puts "#{e.opt}  - #{e.meth_in_bar}" }