I must admit this happens a lot in my kitchen: whenever I need to boil water for tea or coffee, I always fill the kettle up, but never use all the water in the kettle. I don’t usually empty the kettle when I go for that second cup of coffee of the day, I just re-boil the water that was already in the kettle. Well, as it turns out, this is a mistake, much like everything else I do in the kitchen. According to the video below, re-boiling water can harm your health. The chemicals in the water get concentrated and you end up drinking stuff like nitrates which are really bad for you.