You keep two-thirds of your personal water inside the little balloons of your cells. The rest, "extracellular water," is carrying your blood cells around inside your blood vessels or is sloshing around freely amongst your cells.

When you eat a bag of potato chips, a pizza or one-fourteenth of an anchovy, the sodium component of table salt finds its way quickly into your extracellular water. Now it's much saltier outside the cells than it is inside. At the very core of their being, the new sodium and the unsalted water in your cells yearn to be united. But, like disapproving parents, your cell membranes hold the two apart: They admit other water-binding chemicals from better families, but hold sodium at bay.

And in a reckless bid for fulfillment, some water sprints out of the cell through the cell wall and embraces the excess sodium. Ahhhh, your extracellular water feels a returning sense of completion and well-being.

But the cells are now dehydrated -- drying out. Your cells will urge you to drink. And your extracellular water can be depleted, too. Eat a pizza, then run a marathon on a hot day if you want to dry up both cellular and extracellular water. The salty pizza will pull water out of your cells, and then you'll sweat the water and sodium away and start to look like an anchovy.

"People assume they're little clumps of cells," says University of Iowa psychologist Robert Thunhorst. "They're probably neurons [nerve cells], maybe in the liver, that are sensitive to their own size. When they shrivel up, presumably, that sends a nerve signal to the brain. There is also lots of evidence that they exist in the brain itself."

The watchdogs on your extracellular humidity are a happier subject. Two "baroreceptors" (pressure gauges) near your heart take note of changes in the pressure and volume of your blood. If the pressure or volume is low, your blood is dehydrated. The baroreceptors fire off a nerve telegram to the brain, suggesting you should reach for the lemonade.