You need to use included method/hook. It is being run every time given module is included into other class or module, with this class or module for a param. In your case you want:
module Helper
def i_am
"Default code"
end
def self.included mod
mod.extend ClassMethods
end
module ClassMethods
def who_am_i
class_eval(<<-EOS, __FILE__, __LINE__ + 1)
def i_am
"This was executed in class: \#{self.class.to_s}"
end
EOS
end
end
end
class Tester
include Helper
end