gistfile1.rb

module Foo
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def desideratum
      "yup"
    end
  end
end

module Bar
  include Foo
end

puts Bar.desideratum # => "yup"

class Baz
  include Bar
end

puts Baz.desideratum # => undefined method `desideratum' for Baz:Class

Yes, this approach is not recommended anymore for exactly this reason. Instead, to include concerns the recommended approach is to use ActiveSupport::Concern. See fx Yehuda Katz' article about this.