If you’ve ever left a glass of water out overnight, you’ve probably noticed the weird way it tastes in the morning. According to Discovery News, there’s a scientific explanation for the new flavor. As water interacts with the air, it absorbs small amounts of carbon dioxide and forms minuscule portions of carbonic acid. This newly formed acid lowers the water’s pH, making it more acidic and slightly changing the way it tastes. While Discovery does not recommend leaving water out for too long—as it can host mosquito larvae and grow algae—one night won’t hurt.