module UsefulFeatures
  def class_name
    self.class.to_s
  end
end

class Person
  include UsefulFeatures
end

x = Person.new
p x.class_name

# if you want the method on Person too, you’ll
# have to extend
class Person
  extend UsefulFeatures
end
# could have done this, too: Person.extend UsefulFeatures
p Person.class_name

"Person"
"Class"