Ruby lets you add instance variables at runtime

gistfile1.rb

ruby-1.9.2-p290 :001 > class Foo
ruby-1.9.2-p290 :002?>   def bonk
ruby-1.9.2-p290 :003?>     @bonk = nil
ruby-1.9.2-p290 :004?>     end
ruby-1.9.2-p290 :005?>   end
 => nil 
ruby-1.9.2-p290 :006 > f = Foo.new
 => #<Foo:0x007ff15c1e65b8> 
ruby-1.9.2-p290 :007 > f.bonk
 => nil 
ruby-1.9.2-p290 :008 > f.inspect
 => "#<Foo:0x007ff15c1e65b8 @bonk=nil>"