class Money
  def initialize(amount)
    @amount = amount
  end
end

class Integer
  VALID_CURRENCIES = [:kroner, :dollar, :euro]
  def method_missing(m, *args, &proc)
    if VALID_CURRENCIES.include?(m.to_sym)
      Money.new(self)
    else
      super
    end
  end
end

10.kroner
# => en instans av Money
10.kurdere
# => NoMethodError: undefined method ‘kurdere’ for 10:Fixnum