Many of us feel tethered to our water bottles because we've been told that being hydrated is key to being healthy. NPR's Life Kit and Short Wave colleagues teamed up to talk about what science says about hydrating. Turns out much of the lore is due for an update. Example: drinking eight glasses of water a day is good for you. That actually depends on body size and activity level. But Mindy Millard-Stafford, professor in the School of Biological Sciences and director of Georgia Tech's Exercise Physiology Lab, says even mild dehydration, or a two percent loss of water, can impact the brain's executive functions and ability to pay sustained attention.