Water itself is a very simple substance made of hydrogen and oxygen, and in its pure form, it does not spoil or expire. However, the water we drink is rarely completely pure. It usually contains minerals, oxygen, and sometimes small traces of other substances. Over time, these can change the way the water tastes or smells. If water is stored in a container that is not clean or sealed, bacteria or algae can grow, making it unsafe to drink.
If you leave water out in an open glass, dust, microbes, and even tiny insects can get into it. While the water molecules remain the same, the added contaminants can make it unpleasant or unhealthy. That is why it is best to store drinking water in a clean, closed container and use it within a reasonable time.
Knowing whether water can get old depends on how it is stored and what might get into it. For more everyday answers and interesting facts, visit Vero Q&A.