It's to be noted that while Ruby CAN change builtin functions, the metaprogramming ability is much more often used in cases like ActiveRecord, where you can say

It's to be noted that while Ruby CAN change builtin functions, the metaprogramming ability is much more often used in cases like ActiveRecord, where you can say `acts_as_list`, and it reopens the AR class and inserts new list-related functionality. It's more powerful than simply using modules, since it can make decisions as it's being included. Working with Ruby on a day-to-day-basis, I have never reopened core classes and changed anything. Rails comes with some common extensions which simply add new methods, which are less error prone (although I avoid Rails & ActiveSupport whenever possible).