It's old news by now that we don't really need to drink eight glasses of water per day. Nobody really knows where the "8 x 8 Rule" originated, and most researchers agree it isn't supported by science. But don't put down that bottle or cup, yet; there are still plenty of reasons to keep well hydrated, and water is the best option.

1. Water regulates body temperature

Water has "high specific heat," which means it is resistant to changes in temperature, up or down. Body temperature affects the rate of the chemical reactions that keep us going; normal body temperature, which can vary slightly depending on person or mood or time of day, is the most efficient. When it's hot, we sweat, and the evaporating water on our skins cools us. When it's cold, blood vessels near the skin contract, holding blood, and water, closer to our cores to provide more insulation and to prevent heat loss.

2. Water lubricates our joints

Most of our joints contain a dense, egg-yolk-like liquid called synovial fluid, which nourishes cartilage, removes waste, reduces friction between cartilage, and acts as a shock absorber. Elsewhere, the fluid in our organs helps them keep their shape because water is resistant to compression.

3. Water is essential to proper cell function

Water transports all the sugars, salts, fats, amino acids, vitamins, and minerals our cells need. About two-thirds of our water is found inside our cells, which are mostly microscopic drops of water with dissolved ions of sodium and potassium and other elements and larger protein molecules doing this and that. Metabolism, the transformation of food into energy and building blocks, happens in these little drops of water. Afterward, water carries away cell waste and other toxins via the lymphatic system.

4. Water is important to digestion

Digestion begins with the first bite, and water plays a key role throughout, first as enzyme-rich saliva, next as the enzyme-rich mucus covering of our stomach lining, and eventually as the food and enzyme-rich fluid that passes into our intestines.

5. Water is critical to metabolism

Water is the medium within which "metabolic pathways" take place. These step-by-step biochemical reactions transform food into energy and everything else. Hydrolysis is a fundamental metabolic reaction in which complex sugars are broken down into the simpler sugars our cells use by adding hydrogen and oxygen ions taken from molecules of water.

6. Water can prevent hangover symptoms

Alcoholic beverages cause a net loss of water, as alcohol in the bloodstream triggers the pituitary gland to stop producing a hormone called ADH (or vasopressin). A shortage of ADH in the bloodstream tells the kidney to start producing urine, and our cells respond by giving up some of their water. Dehydration may be a big factor why a big headache accompanies alcoholic intake. Drink a glass of water along with your evening glass of wine to keep your body water better balanced.

7. Water helps brain function

Water comprises as much as 75% of our brain cells (about the same percentage as muscle cells) and researchers have noted that dehydration impairs brain function. The electro-chemical reactions that allow our nerves to send and receive signals also require water to function.

8. Water has zero calories (and may have less than zero calories)

A cool glass of water is not the only way to replenish your body's needs, but it's probably the healthiest. Drinks sweetened with sugar are higher in calories and the diuretic effect of caffeinated beverages (such as many sodas and coffee) means replacement isn't one-to-one. And that cold glass of ice water has another advantage — it can help your body burn more calories. The effect is minimal, just a few calories per glass, and the causes are unclear, but over the course of a year, an extra 1.5 liters of water a day can add up to 5 pounds of additional weight loss, not counting extra trips to the rest room. Drink up!