Lesson 15-the riddle of time I. Warm-up Questions Answer 5: It is used under rigorous conditions such as water (especially sea water), acids, dust sudden temperature changes or magnetism W BTL E The end of Warm-up Questions.Lesson 15—The Riddle of Time W B T L E The end of Warm-up Questions. It is used under rigorous conditions such as water (especially sea water), acids, dust, sudden temperature changes or magnetism. Answer 5: I. Warm-up Questions