If you keep a glass of water on the table at night, do not drink it in the morning, better spill it so you will make yourself a favor.
Ordinary water comparing with the food does not contain sugars neither proteins and microbes can easily contaminate it. For the strange taste of water left in the glass during the night, there is an explanation. In fact, given that the water was exposed to the air, it absorbed carbon dioxide and a little part of CO2 is converted to carbonic acid, which means that the water because of the air changed its chemical composition. When carbon acid will release one or two protons will turn into bicarbonate or carbonate, then it lowers the pH of the water to which it also changes the taste.
The next time you get a glass of water from the night before; Recall that was exposed to the bacteria that were inside there is a lot of dust present. That does not mean that the water is polluted, but because of the things that happened overnight do not drink it.