How Rails extends Ruby builtin classes is actually really simple...

modifying-ruby-builtin-classes.rb

# scenario
# ========
# suppose we want functionality like so on any string:
# "racecar".palindrome?  #=> true
# 
# where should we could we add that functionality?

# in ruby, this is surprisingly possible:
class String
  def palindrome?
    self == self.reverse
  end
end

"racecar".palindrome?  #=> true

# notably, this is not completely overridding the
# builtin `String` class in ruby.
# 
# instead, it opens up the builtin String class
# and adds a new instance method `palindrome?` to it,
# thereby enabling all string instances going forward
# to receive the message `palindrome?` and respond accordingly.
# nice.

This is in many cases how Rails extends the functionality of Ruby. For example, the builtin String class doesn't have a blank? method, but in Rails, they do.

(This example was taken from the Rails Tutorial by Michael Hartl.)