extend.rb

class String
  def hello
    puts "hello from String"
  end
end

class MyString < String
  def hello
    puts "hello from MyString" 
  end
end

module StringExtentions
  def hello
    puts "hello from module"
  end
end

s1 = String.new
s1.hello

s2 = MyString.new
s2.hello

s1.extend(StringExtentions)
s1.hello

s2.extend(StringExtentions)
s2.hello