You often find yourself in a situation when you (un)intentionally or (un)consciously reboiling water in the kettle. I understand the reasons, something distracted you while you were making tea, perhaps a long phone call or an urgent meeting at the office. This has already happened to everyone, but next time you sense this familiar situation, stop right there. I warn you not to boil the same water again! The main reason why you should never re-boil the same water again lies in the fact that the composition of the water at boiling point gets altered. That can be very dangerous and you know little about it.