Why does water taste stale after sitting out overnight? Some suspicions I have... 1) Dust settling, and that "stale" taste is dust. 2) Microorganism/bacteria enter and multiply (especially because I drank out of it yesterday, which I suppose would be an easy hypothesis to test... fill the cup but don't drink from it, then wait a day and drink it). 3) It's from a plastic cup, so perhaps a small amount of plastic has leached into the water. I read an answer on answers.com or somewhere and someone said it is because the aeration of the water which has gone away after sitting.