gistfile1.txt

irb(main):001:0> class Foo
irb(main):002:1>   def self.bar; p 'class'; end
irb(main):003:1> end
=> nil


irb(main):008:0> module Foo
irb(main):009:1> def self.bar; p 'module'; end
irb(main):010:1> end
TypeError: Foo is not a module
	from (irb):8

irb(main):014:0> module Foo2
irb(main):015:1> def self.bar; p 'module'; end
irb(main):016:1> end
=> nil