Despite your best intentions, sometimes you might have to buy water in a disposable plastic bottle to quench your thirst. When you've finished the water, what's the harm in refilling the bottle? The reality is PET plastic (stamped No. 1 on the bottom) is not designed to be used over and over again. Once the bottle is open, the sanitary seal is, too. As you drink, the bottle warms up and becomes a home for harmful bacteria. If you want to reuse it, you have to wash it and dry it out thoroughly before you refill, which is nearly impossible. Instead, recycle the bottle and try to remember your reusable bottle for future hydration.