modules_are_weird.rb

module Foo
  
  def instance_foo
    puts "called instance_foo"
  end
  
  def self.classy_foo
    puts "called classy_foo"
  end
end

Foo.classy_foo
# => called classy_foo
Foo.instance_foo
# => NoMethodError

class Bar
  include Foo
end

Bar.classy_foo
# => NoMethodError
Bar.new.instance_foo
# => called instance_foo