The rule of thumb: drink eight ounces of water eight times a day. That’s the best — and only way — to stay appropriately hydrated.

But like many rules of thumb, the eight cups of water eight times a day rule is more myth than fact. The medical validity of the “rule” is as cloudy as the rule’s history.

Let’s start with the history. In September of 2014, economics professor Emily Oster dug into the medical journals in search of the backstory behind the magic number “eight.” She concluded that there’s nothing magical about that number — in fact, it seems somewhat arbitrary. Oster explains in her article:

“The short answer — at least to the specific question of eight glasses versus, say, seven or nine — is no, there is nothing special about eight. This threshold appears to be a long-standing medical myth. It’s not even clear where it started. The best answer I can find (based on this review) is that the source was a 1945 publication by the National Food and Nutrition Board, a government advisory agency, that stated this: “A suitable allowance of water for adults is 2.5 liters daily in most instances. … Most of this quantity is contained in prepared foods.” The theory is that people read this, ignored the last sentence, and the eight glasses a day (about 2.5 liters) recommendation was born.”

More recent scientific exploration has echoed this. In October of 2013, the BBC reported on a study where cyclists were pushed to exercise until they sweated themselves dry — the cyclists weren’t let off the bikes until they had sweated away three percent of their body weight. Then, the cyclists were divided up into three groups and given IV fluids to rehydrate. One group, though, wasn’t rehydrated at all. A second was partially rehydrated, while the third group was fully rehydrated. The cyclists weren’t informed of these grouping differences, (the researchers wanted to avoid the placebo effect), and their post-rehydration performance was then measured and scored. There was no noticeable impact in performance — getting a glass or two of water, therefore, probably could have waited.

So the “eight times eight” rule probably isn’t supported by medical science — but, as the Mayo Clinic notes, it “remains popular because it's easy to remember” and, “for your body to function properly, you must replenish its water supply by consuming beverages and foods that contain water.” Basically, it’s important to stay hydrated, and the “eight times eight” rule can’t hurt.

And if “eight times eight” is too difficult for you, there’s good news: as the Cleveland Clinic points out, our bodies have two ways of telling us its time to drink something. First, there’s thirst — if you feel thirsty, go to the sink and get yourself some water. (The Cleveland Clinic specifically notes that the quality of tap water is “more reliable” than that of bottled water, plus tap water can be filtered to further improve taste and purity.) And second — and these are the Cleveland Clinic’s words, not mine: “Check your built-in water meter. Your urine should be almost clear. If it’s a dark yellow or has a strong odor, drink a couple of glasses of water.” Or maybe just make drinking more water part of your regular routine.