2.2.1 :001 > Integer.frozen?
=> false
2.2.1 :002 > Integer.send(:define_method, :lol) { "azaza" }
=> :lol
2.2.1 :003 >
5.lol => "azaza"
2.2.1 :004 > Integer.freeze
=> Integer
2.2.1 :005 > Integer.send(:define_method, :kek) { "ololo" }
RuntimeError: can't modify frozen class
from (irb):5:in `define_method'
from (irb):5
from /Users/rustam/.rvm/rubies/ruby-2.2.1/bin/irb:11:in `<main>'