Take a look around. Right this very second there's a water bottle within a 10-foot radius of where you're sitting. How did it get there? A drop of physiological need mixed with an ocean of marketing savvy. Water used to fuel the office water cooler; now it's practically become an anytime-anywhere fashion accessory. Fashionistas — not to mention the mailman, the grocery clerk, your yoga instructor, and the school nurse — can all be seen carrying their bottles and Camelbacks of purified, fortified, and natural spring. And whether you're staying hydrated during a marathon shoe-shopping session or drinking from your home Brita filter, there is no doubt about it: Water, water — it's everywhere. But as with any mainstream megatrend, you have to stop and ask yourself, "What's really going on here?" When water bottles become must-have equipment on a planet that's 70 percent liquid, some of the reasoning is bound to be bunk.

Myth: You need to drink eight glasses of water every day.

Fact: No one's sure where the so-called 8-by-8 rule came from, says Heinz Valtin, M.D., a Dartmouth College medical professor and author of two studies on the origin of the theory that the human body works best on eight 8-ounce glasses of water a day. The truth is, your daily requirement depends on your diet, size, and unique body chemistry. To determine how much water you should drink, weigh yourself each morning for 3 to 4 days in a row — pick a time other than your period to rule out hormone-induced water retention. If you lose a whole pound in a day, it means you came up short on liquids the day before. Drink a pint of water or juice first thing in the morning for every pound you've lost and adjust your daily intake until your weight is steady.

Myth: Drink only when you're thirsty and you'll get all of the fluids you need.

Fact: Sedentary folks might do fine using this mantra, but anyone who occasionally feels the urge to be active need not subscribe. "Exercise blunts your thirst mechanism," says Leslie Bonci, R.D., director of sports nutrition at the University of Pittsburgh Medical Center. "You lose fluid so rapidly that the brain can't respond in time." In fact, a recent study from Maastricht University in the Netherlands found that women lose more water during exercise than men. An hour before you hit the gym, grab an extra 20 ounces to hydrate before you dehydrate. "It takes 60 minutes for the liquid to travel from your gut to your muscles," Bonci says.

Myth: Tea and coffee dehydrate you.

Fact: Down two venti house blends and you'll visit the ladies room often enough to earn a VIP pass. But despite its speedy exit, the liquid in your favorite morning caffeine boost still counts toward your hydration goal. After all, it's basically water, unless you muck it up with flavored syrups or dairy. "Caffeinated beverages do not dehydrate you when consumed in moderation, that is, five cups or less per day of coffee, tea, or cola," says Lawrence Armstrong, Ph.D., professor of kinesiology at the University of Connecticut and author of Performing in Extreme Environments. In fact, Dr. Armstrong says that any fluids you ingest will help keep your cells saturated, including juice, iced tea, or soda. (Just keep an eye on the calorie count in order to wet your whistle without widening your waistline.)

Myth: Bottled water is better than tap.

Fact: Unless you're traveling abroad or south of the border, what comes from the kitchen sink is as nutritious as water gets. Tap water is chock-full of minerals, such as sodium, calcium, magnesium, and zinc, that remain even when flushed through a filter or poured into a bottle. Purified and distilled waters, however, are boiled during processing to strip them of any trace minerals. Store-bought H₂O also lacks the fluoride that's sprinkled into the water supply to keep your teeth healthy. If you're hooked on the bottle, skip brands that tout the words "distilled" or "purified" on their labels unless they're refortified with minerals. Look for roughly 25 percent of your Recommended Dietary Allowance of calcium and up to 200 milligrams of magnesium on a bottle's nutrition panel.

Myth: Drinking water before a meal helps you lose weight.

Fact: The water you drink before or during a meal won't keep you from overeating, and it won't flush food out faster from the body, says Barbara Rolls, Ph.D., author of The Volumetrics Eating Plan. "Water doesn't bind to the food, so it empties out of the gut really quickly," she says. You can pad your meals with water in other ways to help cut calories. When water is contained in foods like vegetables, it travels through the stomach and into the intestines along with the rest of the meal, making you feel full without adding to the meal's calorie count. "If you just drink water, you will only satisfy thirst mechanisms, whereas foods that contain a lot of water satiate hunger and hydrate you as well," Dr. Rolls says. Choosing broth-based soups like chicken noodle or especially juicy fruits and vegetables like watermelon, peaches, cucumbers, and tomatoes is an easy way to fill up on water.

Myth: Vitamin water is better for you than plain.

Fact: Fortified water might pack a concentrated punch of a nutrient like Vitamin C, but it won't make its way into your bloodstream any faster than a standard supplement would, Bonci says. "Plus, most vitamin waters are not fortified with the whole balance of vitamins that the body needs." (Not to mention the unnecessary sugar that often contributes to their taste.) To mimic the fruity flavor without doing dietary harm, mix a splash of your favorite juice with water or seltzer.

Myth: Marathon runners need sports drinks; mere mortals do not.

Fact: Gatorade was invented to help football players stay in top form during sweaty outdoor games, and it can do the same for you on a sweltering afternoon at the beach. When you sweat a lot, you lose both salt and water through your pores, Bonci says. Sport drinks can replenish your supply of both. "The sodium in sports drinks also helps your body retain more fluid," Bonci says, so you can chug less and feel better fast. Drink yours ice cold and it will make you feel cooler, too, by drawing heat out of your tissues on its way down your gastrointestinal tract.

Myth: I can't get dehydrated while swimming.

Fact: You're actually more likely to become dehydrated when you spend an extended period of time in the pool or the ocean, Dr. Armstrong says. "Part of the reason is psychological; when you come out of the pool, the last thing you want to look at is a glass of water," he says. But physiology also comes into play. "Thirst is controlled by the volume of blood at the center of the body," Dr. Armstrong says. So when the brain senses a lack of blood at your core, you reach for your glass. But water — in the pool, not the bottle — creates a hydrostatic pressure that pushes blood from your skin to the center of your body, tripping up the system.