singletons.rb

# self

class Foo
  # self
  # belongs_to
  def Foo.foo
    @bla = '123'
    puts 'bla'
  end

  def Foo.bar
    @bla
  end

  def bar

    # self is instance of foo
  end

  class << self
    def foo2

    end

  end

  def call(bla)
    puts bla
  end
end

string.rb

a = "Kostas"

puts a.reverse
puts a.length

class << a
  def reverse
    'bla'
  end

  def length
    'computers says no'
  end
end

puts a.reverse
puts a.length