x.rb

Greeter = Object.new

class << Greeter
  def greet(name)
    "HELLO, #{normalize(name)}!"
  end

  private

  def normalize(name)
    name.strip.upcase
  end
end

class Person
  def initialize(age)
    @age = normalize(age)
  end

  def greet(name)
    Greeter.greet(name)
  end

  private

  def normalize(age)
    [age.to_i, 25].min
  end
end

person = Person.new(12)
p person.greet("Joe")