It condenses and evaporates to nothing. As a result, your kettle or pot may burn on the bottom or get distorted. If you don't catch the kettle before the water runs out, it could smoke up your house, setting off the smoke alarm. Worse, if pieces of the pot melt, it could start a fire. You may find stories on the internet claiming that boiling water for an extended period can render it hazardous. According to these sources, reboiling water or boiling water for an extended period causes minerals and metals to concentrate, which can contribute to cancer if taken in significant numbers. These statements, however, have been proven to be erroneous. While it is true that boiling water over an extended period might cause a buildup of toxins, it would take hundreds of boilings for this to occur. So there's no need to toss out your water if you've boiled it for more than three minutes.