Water can smooth stones and carve canyons. But don't count on drinking lots of it to help you lose weight, cure headaches or make you look younger, say two kidney specialists who are fed up with the "urban myths" surrounding excessive hydration.

Healthy adults, for example, don't actually need to consume eight glasses of water a day. Scientists have no clue where the dictum originated; not a single study explains why eight might be better than four or 10.

Moreover, drinking "lots" of water apparently is a waste of time (and money if you're a fan of bottled water) unless you're an athlete or live in a hot climate. Water may be more essential to life than oil, but tanking up doesn't necessarily curb hunger pangs, reduce headaches, improve skin tone or keep organs healthy by flushing out toxins, the University of Pennsylvania researchers said in an editorial in a medical journal.

"Our purpose was to relieve people of the burden that they have to drink extra water in order to be healthy," co-author Stanley Goldfarb told me. "There's no evidence of that."

This is what happens when scientists try to prove common sense. Will the number of breaths we need to take be studied next?

Although there's no data showing that excess water can make you healthier, the authors admit there's no evidence of a lack of benefit, either, which suggests that the "burden" on us really isn't that great.

When I get a mild headache, I drink water and it usually goes away. My skin looks better when I'm adequately hydrated. And when I'm hungry, drinking water makes me feel full, so I tend to eat less.

Anecdotal evidence, I know. But sometimes it's better to make a health decision based on what your body tells you rather than on studies, especially when many of them are subject to errors, ghostwriters and industry bias.

Water is virtually free. Unlike every other beverage, it has no calories, artificial colors or additives. It has no negative effects, unless you deliberately hyperhydrate.

And because everyone has different hydration needs due to different climates, sizes, exertion and clothing, we really don't know how much is "too much."

Water certainly is not a cure-all; too many runners are saddled with hydration belts, and there's no need to be constantly gripping a plastic water bottle. But its potential healing benefits shouldn't be discounted.

Only one small trial with 15 people has looked at whether it can relieve headache pain, and the results, while "not statistically significant," actually were encouraging. Suffering migraines rather than milder types of headaches, the participants who increased water intake experienced fewer headaches than those who did not.

Water also may — or may not — help you excrete toxins. "Drinking a lot of water very quickly tends to lower blood flow to the kidney," Goldfarb said. "That actually impairs its ability to excrete toxins." But researchers also have found that drinking water does have an impact on clearing various substances by the kidney. The problem is that these studies don't indicate whether a clinical benefit might result.

And when it comes to weight loss, it's possible that the water you drink before you eat would stay in the stomach, a small-volume area, and suppress your appetite, Goldfarb said. But drinking water with meals didn't seem to have the same effect.

What they didn't comment on is whether water can help with weight loss by displacing milk, juice or soft drinks.

Water consumption is elementary: Drink it if you're thirsty and if your urine is dark yellow. The exception is if you're elderly, because the sense of thirst diminishes with age, said Alexa Fleckenstein, co-author of "Health20 — Tapping into the Healing Power of Water." She recommends that the elderly should actually count their glasses.

But how many? Fleckenstein likes seven because there's a sacred quality to the number. That's hardly a scientific reason. But maybe we don't need one.