Normally, you'd just keep the water and use it for something else later. Either you let it sit in the kettle or keep it in a pot on the stove. Sounds perfectly harmless, right? Nope, in fact it is the exact opposite.
Let's take a closer look at what happens when water boils. At first glance, you see it bubble and steam, but there is also something happening at the chemical level. By cooking the same water multiple times, the oxygen content changes and potentially dangerous substances accumulate. You can really taste this with tea: Tea brewed with fresh water tastes very different to water that's been re-used.
Boiling water is a great way to kill bacteria, but when you boil water over and over again, dangerous gasses and poisonous substances like arsenic, nitrates, and fluoride can form as a result of the changed chemical structure. Various studies have shown a connection between this fluoride and the damaging effects it has on the nervous system and brain.
After this whole process, it really isn't just drinking water anymore and the minerals that are actually good for you, like calcium, can create problems and cause kidney stones to form.
So this is the reason why you really shouldn't ever boil the same water twice. Obviously, it is most harmful when done in large quantities. Smaller amounts for your tea isn't really a huge problem, but maybe start using that extra water to give your houseplants a drink instead - it isn't dangerous for them!
Interesting. You really have to be careful if you are constantly reusing old water. It's good to know that it's not as harmless as you perhaps otherwise thought.