But what does that mean, exactly? Here's a pretty good article on hydration and here's a much more detailed article for you hydration scholars out there, complete with tables showing water content of foods.
To test your hydration level:
Pinch a fold of skin on the back of your hand, forearm or abdomen, and see how long it takes to resume it's former position. If a ridge stays elevated, you're dehydrated - hit the water bottle!
Also- the color of your urine is a good marker. Aim for a lemonade color - dark or brown urine is a sign of dehydration, and completely colorless urine may indicate excessive hydration.
Remember that alcohol is dehydrating, as is the sun, and so is sweating, so drink plenty of water if you're boozing it up in the summer sun while playing beach volleyball (Personally, I miss those days!).
Pay special attention if you're older, if you take a diuretic ("water pill"), if you have a fever, or you have diabetes, as you can be more prone to dehydration.
Can You Drink Too Much Water?
It's possible to overhydrate, too, although that generally only happens to people with severe kidney or liver disease or in extreme athletes who don't perspire enough.