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You’ve probably heard that we’re mostly made of water, which is true. Up to 60% of the human adult body is water. The largest share belong to the brain and heart, which are composed of 73% water, and the lungs, which are made of about 83% water. Even the bones are watery — about 31%.

Given these facts, it makes sense that you have to stay well hydrated to live healthily. But how much water should you drink? One of the most common guidelines suggests each person should drink at least eight 8-ounce glasses of water per day, which total about 1.9 liters (64 ounces) of water. This is referred to as the “8×8 rule”. But the truth is that every person is different, which includes their water requirements.

Why water is important for life

Our planet is blessed with bountiful amounts of water. From polar ice caps to some of the harshest deserts, water is virtually everywhere, including in the air we breathe. And where there’s water, life seems to have found a way to exist and proliferate.

The reason why life and water are so inexorably linked has to do with the molecule’s unique chemical properties. Water can dissolve nearly anything and stays liquid at most temperatures found on Earth. Because it flows, water is the perfect medium to transfer substances — both nutrients and waste — between a cell’s membrane and the environment. It’s thanks to the way the molecule’s two hydrogen atoms and one oxygen atom are bonded together that makes water a universal solvent. Water has polarity, meaning the hydrogen side is positively charged while the oxygen end is negatively charged. The positive region tends to attract negative ions while the negative end lures in positive ions. This makes it perfect for transporting essential substances for life such as phosphates or calcium ions into and out of cells.

In complex organisms, water plays numerous crucial roles such as regulating internal body temperature through sweating and respiration, flushing waste through urination, absorbing shock for the brain, spinal cord, and fetus, forming saliva, and lubricating joints. Water is also important for metabolizing and transporting carbohydrates and proteins, which the body uses for energy.

When people aren’t properly hydrated, things start to break down. Studies have shown that mild dehydration (1-3% of body weight) can cause headaches, impair mood and concentration, reduce physical performance, and impact other important aspects of brain function. Conversely, a well-hydrated body offers protection against constipation, the formation of kidney stones, acne, and even bladder and colorectal cancer.

Some warning signs that you’re not getting enough water include weakness, low blood pressure, dizziness, and confusion. Dark-yellow or orange urine is usually a sign of dehydration — it should be pale-yellow or colorless.

How much water should you drink then?

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Generally, each person should strive to replenish as much water as they lose daily through excretion, perspiration, and other bodily functions. However, this will vary from person to person. If you live in a tropical environment or work a physically draining job, you’ll naturally lose a lot more water by sweating than those living in a chilly climate or who work office jobs. Diet also plays a major role since we get a lot of water from food. Fruit such as watermelon and strawberries can be as much as 90% water by weight and according to a 2004 study, 20% of a person’s daily water intake comes from food.

Some like Jurgen Schnermann, a kidney physiologist at the National Institutes of Health, claim that many of us could cover our bare-minimum daily water needs simply by ingesting food, without having to drink anything during the day (although he’s not making such a recommendation).

The fact of the matter is that the 8×8 rule, like most other recommendations, are just guidelines.

What’s more, the idea that one must specifically drink water, rather than caffeinated drinks such as coffee, tea, and soda, is severely flawed. According to a 2015 article in Harvard Men’s Health Watch, while it true that caffeinated beverages or those containing alcohol are dehydrating because they make you urinate, the water from these beverages still leads to a net positive contribution.

What’s more, drinking too much water — as some self-professed health gurus advise — can lead to all sorts of trouble. Water intoxication can cause sodium in the blood to reach abnormally low levels. Although such cases are rare, some athletes and people using ecstasy are known to have died due to too much water intake that led to hyponatremia (low sodium levels in the blood).

People who are sick and the elderly tend to have a poorer thirst response, which means they might need to be extra careful. But at the end of the day, the best advice is to listen to your body. After all, we feel thirsty for a reason. There’s no need to force yourself to chug copious amounts of water just to satisfy some arbitrarily rigid rule.