You might have noticed that water can taste a bit off sometimes, especially if you leave a glass or open bottle out over night. But is that odd taste is an indicator that the water has gone bad? That's hard to answer, Esther Inglis-Arkell explains for io9

When you drink from a glass, you introduce microbes from your lips, mouth and saliva into the water and its container, Inglis-Arkell writes. “They’ll often be joined by whatever is riding around on the dust swirling in your house,” she notes. These microbes beget more microbes in environments with balmy temperatures. The colder the room the longer the bottle or glass of water will keep.

Most tap and bottled water also contains low levels of chlorine compounds designed to keep microorganisms at bay. But, the chemicals can only succeed for a day or two, Kellogg Schwab, an environmental microbiology at Johns Hopkins University, told Time’s Markham Heid in 2014.

Interestingly, the microbes aren’t actually behind the stale taste, as Inglis-Arkell points out. That’s carbon dioxide, which dissolves in water to form carbonic acid. “Essentially, after water has been out too long, you’re drinking very mild acid rain,” writes Inglis-Arkell.

That’s when water is exposed to the outside world, but what if it’s sealed in a bottle? As Smart News writer Rose Eveleth noted back in 2013, bottled water itself doesn't go bad, though bottled water does carry an expiration date. Plastic and environmental chemicals can sneak in. Some are dangerous, some aren’t.

Before you freak out about some day old water, rest assured. It's almost certainly safe to drink, Kellogg said. You might want to avoid reusing dirty glasses, sharing glasses with others or touching the rim of your glass with dirty fingers though.