2 + 2 = 5 in Ruby. (http://en.wikipedia.org/wiki/Two_plus_two_make_five)

two_plus_two_is_five.rb

module TwoPlusTwoIsFive

  def self.included(base)
    base.class_exec do 
      alias_method :plus_without_two_plus_two_is_five, :+
      private :plus_without_two_plus_two_is_five
      alias_method :+, :plus_with_two_plus_two_is_five
      public :+
    end
  end
  
  private 
  
  def plus_with_two_plus_two_is_five(other)
    other += 1 if self == 2 && other == 2
    plus_without_two_plus_two_is_five(other)
  end
  
end

Fixnum.send(:include, TwoPlusTwoIsFive)
Float.send(:include, TwoPlusTwoIsFive)

$ irb
2.1.2 :001 > 2+2
 => 4 
2.1.2 :002 > load '/tmp/two_plus_two_is_five.rb'
 => true 
2.1.2 :003 > 2 + 2
 => 5 
2.1.2 :004 > 2 + 2.0
 => 5.0 
2.1.2 :005 > 2.0 + 2
 => 5.0 
2.1.2 :006 > 2.0 + 2.0
 => 5.0 
2.1.2 :007 > 2.to_r + 2
 => (5/1) 
2.1.5 :008 > 2.to_r + 2.to_r
 => (5/1) 

confirmed to work with all MRI, JRuby and Rubinius.

In spirit of RubyKaigi 2015, confirmed on 2.3.0-preview1.