dummy.rb

class TestClass
  def self.some_other_method
    puts "Hi"
  end

  def my_instance_method
    puts "My instance method"
  end

  # Also possible and convenient when you
  # have to define many class methods.
  class << self
    def first_method
      puts "FIRST"
    end

    def second_method_etc
      puts "SECOND"
    end
  end
end

i = TestClass.new
puts i.my_instance_method # Works

TestClass.some_other_method # Works
TestClass.first_method # Works

TestClass.new.first_method # Fails
i.first_method # Fails